Distilled water by definition is neutral - ph 7.0, neither acidic, nor basic.

During bottling and handling it can, and often does, get "contaminated" with CO2 from the air. That exposure forms a very mild carbonic acid, which is largely harmless - even to wool.

That exact same contamination occurs in mineral water, as well. So, with mineral water you get two kinds of contaminants - minerals and carbonic acid.

Rowenta recommends against distilled water, because their heating element is coated with some substance that enhances steam production. Distilled water in some way unknown to me, and also to the representative with whom I spoke, interferes with that process. Perhaps someone from Rowenta will weigh in and inform us about the chemistry involved.

Most iron manufacturers, today, state that it is unnecessary to use distilled water, but only because their irons filter the minerals out of the tap water.