Distilled vs Deionized Water: Differences, Uses, and Which to Choose

Distilled and deionized water look identical, cost about the same, and both are sold as "pure water," so it is easy to assume they are interchangeable. For many jobs they are. But there is one difference that matters more than any other and that most comparisons skip: distilled water is made by boiling, which kills microbes, while deionized water is made at room temperature and does not remove bacteria at all. That single fact decides whether the water is safe for a wound, a CPAP machine, or your mouth, even though a purity meter might rate them the same. This article explains how the two are made, how they really differ, and exactly which one to use for labs, car batteries, coolant, medical devices, and drinking.

The water in which dissolved minerals, salts, organic matter, ammonia, CO2, and other impurities are removed is called distilled water. It is produced by the distillation process and is pure and more expensive than deionized water.

The water in which all the dissolved ions and minerals are removed is called deionized water. Deionized water (DI) is also called demineralized water (DM). It is produced by the deionization process and is cost-effective to use in general laboratories when high-grade pure water is not required. Some of the differences between them are given below:

Figure: Distilled water vs deionized water

Distilled vs deionized water: quick comparison

The rest of this article explains each of these rows, and gives a plain "which one should I use" guide near the end.

What is distilled water?

Distilled water is water that has been purified by distillation: it is boiled to steam, and the steam is then cooled and collected as liquid. Because the minerals, salts, and most microbes and organic matter are left behind when the water evaporates, the collected water is very pure. It removes dissolved minerals, salts, and, because boiling kills them, most bacteria and fungi. A small amount of volatile organic compounds can evaporate along with the water and carry over, so distilled water is not perfectly free of every organic, but it is both mineral-poor and largely microbe-free.

What is deionized water (DI water)?

Deionized water, often written DI water and sometimes called demineralized water, is water from which the dissolved charged particles (ions) have been removed. It is made by passing water through ion-exchange resins, which swap the mineral ions in the water (such as calcium, magnesium, chloride, and bicarbonate) for hydrogen (H⁺) and hydroxide (OH⁻) ions that combine to form more water. With almost no ions left, DI water barely conducts electricity, which is how its purity is measured.

The crucial limitation: deionization only removes charged ions. It happens at room temperature and does nothing to bacteria, viruses, or uncharged organic molecules. So DI water can be extremely pure by an electrical measurement and still contain living microbes. Deionization is often combined with other steps (reverse osmosis, UV, and fine filtration) when microbe-free water is actually needed, but ion exchange by itself does not sterilize.

How distilled and deionized water are made

Distillation. Water is boiled, and the steam is led away and condensed back into liquid in a separate container. Because water boils at a lower temperature than most minerals and salts, those impurities stay behind, and the condensed water is much purer. For very high purity, water can be double- or triple-distilled (distilled two or three times). Distillation is effective but slow and energy-intensive, because it requires heating all the water to boiling.

Deionization (ion exchange). Water is passed through beds of ion-exchange resin. Cation resins swap positive mineral ions (Ca²⁺, Mg²⁺, Na⁺) for H⁺, and anion resins swap negative ions (Cl⁻, HCO₃⁻, SO₄²⁻) for OH⁻; the released H⁺ and OH⁻ combine into water. This is faster and cheaper than distillation and removes ions very effectively, but it removes only ions. (Reverse osmosis is a separate purification method, sometimes used alongside deionization, that forces water through a fine membrane to remove a broad range of contaminants; it is not part of ion exchange itself.)

The difference that matters most: microbes

This is the difference most water comparisons overlook, and it is the one this site is best placed to explain.

Distillation kills microbes. Distillation works by boiling. Boiling kills most bacteria and fungi, and the microbes are also left behind when the steam rises. So distilled water is not only mineral-free but largely free of living organisms.

Deionization does not. Ion exchange happens at room temperature and targets only charged ions. It has no effect on bacteria, viruses, or organic molecules, which have no net charge for the resin to grab. Worse, the moist resin beds and storage tanks of a deionizer can actually harbor and grow bacteria, which then shed into the water. So DI water can read as ultrapure on a conductivity meter and still be microbiologically contaminated.

Why this matters: purity is not one single thing. Deionized water is controlled for ions; distilled water is also low in microbes. A low conductivity reading proves the water is low in ions, not that it is safe from bacteria. This is why DI water, however "pure" it measures, must not be assumed sterile or safe for wounds, medical devices, or drinking, while distilled water is the safer choice wherever microbes matter.

Is distilled or deionized water corrosive?

Both distilled and deionized water are often called "aggressive" or "hungry," and the reason is the same for both: they are stripped of ions. Pure water is not an acid and is not corrosive in the way a strong chemical is. But because it contains almost no dissolved ions, it tries to reach a new balance by dissolving whatever it touches. It will leach metal ions (iron, copper, lead, zinc) from pipes and containers, which is why high-purity water is stored in glass or special plastics, not ordinary metal.

Pure water also absorbs carbon dioxide from the air, which dissolves to form weak carbonic acid and drops the pH from a neutral 7.0 toward about 5.5. This mild acidity adds to its tendency to attack metals over time.

This "hungry" behavior applies to both distilled and deionized water, because both are ion-depleted; it is a property of pure water, not of one type specifically. The practical lesson is about storage and contact: keep either type away from reactive metals, and use appropriate containers, especially for long-term contact.

Purity grades: from single-distilled to ultrapure

Purity is a spectrum, not a single level.

• Single, double, and triple distilled: distilling water more than once removes more impurities each time. Triple-distilled water is very high purity, used where even trace contaminants matter.
• Laboratory water types (I, II, III): labs classify pure water into grades. Type III is basic purity for rinsing and feedwater. Type II is general analytical grade for making buffers and media. Type I is ultrapure (around 18 MΩ·cm resistivity), used for the most sensitive work such as HPLC, PCR, and cell culture.
• A key caveat: deionization alone can reach Type I resistivity (very low ions) but not true Type I purity, because it does not remove microbes, particles, or all organics. Genuine ultrapure water combines deionization with reverse osmosis, UV treatment, and fine (0.2 micron) filtration. Resistivity alone does not prove water is ultrapure.

Can you drink distilled or deionized water?

Neither distilled nor deionized water is a good choice for everyday drinking, for two different reasons.

Both lack minerals. Ordinary drinking water carries dissolved minerals such as calcium, magnesium, and potassium that the body uses, and that also give water its taste. Purified water has almost none. Drinking only mineral-free water over a long period provides none of these and, because pure water is "hungry," can even draw some electrolytes out of the body. This is why distilled and deionized water taste flat and are not recommended as your main drinking water.

Deionized water has an added risk: microbes. Because deionization does not remove bacteria or viruses, DI water is not sterile and should not be assumed safe to drink at all, especially from an industrial or laboratory system that was never designed or monitored for drinking-water safety. Distilled water, being largely microbe-free, is safer in this respect, and small amounts (as in a distilled-water product sold for consumption) are not harmful, but it still lacks minerals.

The short answer: an occasional sip of distilled water will not hurt you, but neither type is meant for regular drinking, and DI water in particular should not be treated as drinking water.

Which one should you use?

For many jobs, distilled and deionized water are interchangeable, because both are mineral-free and both prevent the scale and deposits that ordinary water leaves behind. The choice matters most when microbes matter. A quick guide:

• Laboratory (general use, rinsing, making solutions): either works; deionized is common because it is cheaper. For sensitive analytical work (HPLC, PCR, cell culture), ultrapure water is needed (see purity grades below).
• Car batteries (lead-acid): deionized (or distilled) water. Both are mineral-free, which prevents the mineral deposits and conductivity problems that shorten battery life. DI is commonly sold for this.
• Engine coolant / radiators: distilled or deionized, both fine. Mineral-free water avoids scale in the cooling system.
• Steam irons and humidifiers: distilled is usually recommended to avoid mineral scale (check the appliance manual; some now advise ordinary water).
• CPAP machines: distilled water. This is a device that turns water into breathable vapor, so microbial safety matters; distilled is the standard recommendation, and DI is not a substitute.
• Medical uses (wound cleaning, irrigation, dentistry): distilled or purpose-made sterile water, never plain DI water, because DI is not microbe-free.
• Aquariums: either can be used as a base, but minerals must be added back for the fish; this is about mineral content, not microbes.
• Drinking: neither is a good everyday choice (see below), but if forced to choose, distilled is the safer option because DI is not microbe-controlled.

The single rule that covers most decisions: if microbes matter, choose distilled; if only minerals matter, either works and DI is usually cheaper.

Why not just use tap water?

Tap water contains dissolved minerals and organic compounds that can interfere with laboratory work: the ions can cross-react and produce false results, and the calcium and other minerals can deposit on and corrode metal parts and equipment. That is why labs use distilled or deionized water rather than tap water for making solutions, rinsing glassware, and running instruments.

How to remember

Distilled boils, deionized swaps. Distillation boils water and catches the steam (so it kills microbes). Deionization swaps ions on a resin at room temperature (so it does not). The method tells you everything about what each removes.

DI kills nothing. Deionized water can be spotless on a purity meter and still be full of bacteria, because ion exchange removes only charged ions, not living things. Low conductivity is not the same as sterile.

If microbes matter, distill. The one rule that settles most decisions. CPAP, wounds, anything medical: distilled. Batteries, coolant, lab rinsing: either, and DI is cheaper.

Pure water is hungry. Both types, stripped of ions, dissolve whatever they touch and pull CO₂ from the air. Store them in glass or proper plastic, not reactive metal.

Key exam facts

Where students get confused

"Are distilled and deionized water the same thing?" No. They are both very low in minerals, so they behave similarly in many uses, but they are made differently and differ in one big way: distilled water is largely microbe-free (it was boiled), while deionized water is not (ion exchange does not remove or kill microbes). Same mineral purity, different microbial purity.

"Is deionized water sterile because it is so pure?" No. "Pure" here means low in ions, measured by conductivity. Deionization does nothing to bacteria or viruses, and resin beds can even grow them. Low conductivity does not mean sterile.

"Which is more corrosive, distilled or deionized?" Neither is meaningfully more corrosive than the other. Both are "hungry" because both are ion-depleted, so both leach metals and absorb CO₂. It is a property of pure water in general, not of one type. (The old idea that DI is specially corrosive is misleading.)

"Can I use deionized water in my CPAP or on a wound?" No. Use distilled (or purpose-made sterile) water. DI water is not microbe-free, so it is not safe for anything that goes into the airway or a wound, even though it may be very pure by conductivity.

"Is DI water the same as reverse osmosis (RO) water?" No. Deionization uses resins to swap ions; reverse osmosis forces water through a membrane to remove a broad range of contaminants. They are different methods and are often used together in high-purity systems.

References

1. Clinical and Laboratory Standards Institute (CLSI). Preparation and Testing of Reagent Water in the Clinical Laboratory. CLSI guideline GP40. 4th ed. Wayne, PA: CLSI.
2. ASTM International. Standard Specification for Reagent Water (ASTM D1193). West Conshohocken, PA: ASTM International.
3. Tille PM. Bailey & Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
4. Reybrouck G. Water quality in the laboratory. In: Russell, Hugo & Ayliffe's Principles and Practice of Disinfection, Preservation and Sterilization. 5th ed. Oxford: Wiley-Blackwell; 2013.