Yesterday at the supermarket there was de-ionized water (for household irons) on sale at 1/5Litre, about half the price I'm used to. I was about to grab a few, but then reading the label gave me pause: "not fit for drinking", "keep out of reach of children". So, what stuff could there be in that water that is unfit for humans (apart from the fact that it's not good to dring mostly water that is too soft)? and could it be unfit for photo chemistry? Notes: - I use (so far) de-ionized water (a) for final negative rinse because tap water contains particulate matter and I've not gotten around to install an in-line filter (b) for mixing negative developer, just to be on the safe side. - Here I'm talking about de-ionized water for household irons, not the high-tech stuff used in semiconductor fabrication labs. - Where I live (France) de-ionized water is a commodity, normally at 2 for a jug of 5 liters. I never saw distilled water on store shelves. From previous threads on related subjects, it seems that the situation in the US is different, with distilled water being available and cheap. - Valuable insight is given by dancqu in the thread www.apug.org/forums/forum37/38455-distilled-water-question-4.html (emphasis added): But that does not really answer my question.