Distilled and deionized water are frequently treated as interchangeable because both have very little dissolved content, and the processes behind them are entirely different in a way that matters. One separates by evaporation, the other by ion exchange, and the difference shows up in what each one fails to remove.
How each is produced
Distillation boils water and condenses the vapour, leaving dissolved solids, most metals and anything else non volatile behind in the boiling chamber. Deionization passes water through cation and anion resins that exchange dissolved ions for hydrogen and hydroxide, which combine into water, so the ions are removed rather than substituted. Distillation is slow and energy intensive; deionization is fast and consumes resin that must be regenerated with strong chemicals or replaced.
What each one misses
This is the important difference. Deionization removes charged species and nothing else: bacteria, viruses, particles and many organic molecules pass straight through, because there is nothing for the resin to exchange. Distillation removes almost everything that will not evaporate, including organisms, but lets through compounds that boil at or below water's boiling point and travel with the steam, which is why good distillers include a vent or a carbon post filter.
Which is used where
Deionized water is the technical choice where dissolved ions are the problem and biological purity is not: car washing, window cleaning, aquariums built up from a blank base, and laboratory work where the system is managed properly. Distilled water is what households buy for steam irons, humidifiers, CPAP machines and batteries, and what is specified where a broadly clean water is wanted without on site resin management. The two are also combined, with reverse osmosis first and deionization polishing, in demanding applications.
Drinking either
Both taste flat, because the USGS describes hardness as dissolved calcium and magnesium and both processes remove them along with everything else. Neither is harmful. The practical caution applies to deionized water from a technical unit, where the resin bed is a wet, warm environment that can support bacterial growth and nothing in the system was designed to make water safe. For everyday drinking, reverse osmosis with remineralization is the sensible household choice. Our softener guide covers hardness across the house.
Questions people ask about distilled vs deionized water
Are distilled and deionized water the same?
No. Distillation evaporates and condenses; deionization exchanges ions on resin. They differ most in what they fail to remove.
Which removes bacteria?
Distillation, since organisms do not evaporate. Deionization removes charged ions only, so organisms pass straight through.
What does distillation miss?
Compounds that boil at or below water's boiling point and travel with the steam, which is why a carbon post filter is often fitted.
Can I drink either?
Both are safe in themselves and taste flat. Water from a technical deionizer should not be drunk, since nothing in it was designed for safety.