Question:
I know everyone recommends using distilled water in a humidifying device,but...

Can reverse osmosis water be used as well? The RO process supposedlyremoves all chemicals and minerals from the water.

Answer: In a word: Yes.

Now for my LONG post (you've been forewarned):

Reverse osmosis is a very good method to remove harmful compounds from thetapwater. In this process water is filtered through an ultra fine membraneat a pressure that is higher than the osmotic pressure. The membrane keepsback most of the organic and inorganic compounds dissolved in the water, butallows the water molecules to pass. A reverse osmosis system thus removes(except for a very small amount that remains) all unwanted substances likenitrate, phosphate, silicate, aluminum, copper, arsenic, asbestos, atrazine(herbicides/pesticides), lead, mercury, etc. The amount left behind in onthe order of 20 ppm TDS and is acidic (pH of ~6). It is generally acceptedthat porosity of the membrane is very tight with holes (if you want to callthem that) of 5 to 20 angstroms (.00000002 inches in diameter). Themembranes are several mils thick with most consisting of an asymmetric layer(active membrane) with the balance a more porous substrate, spongy in naturethat serves as the support for the asymmetric portion.

The quality of the water is so pure that it is used in photographychemicals, kidney dialysis, humidification, battery water, hermodialysis,and various biomedical and laboratory applications. Rest assured, thiswater is good stuff. At a price of (MAX) NINE cents per gallon (thatincludes maintenance and water costs), it would be cost effective if youwere to purchase an RO unit to also use as an everyday drinking watersource. Home distillers OTOH are a waste of money and can be potentiallydangerous as rust in a major problem in those systems.

Distilled water on the other hand is ~98% pure (slightly higher than ROalone). The difference is between .5% and 3% purity. If you already own anRO unit and maintain the filters, this is more than adequate for a humidor.One side note, does anyone really know how "sterile" their distilled wateris when purchased from the store?

From the Journal of Hospital Infection:

We examined the following samples of water from 10 hospitals for microbialcontamination: water obtained using an ultra filtration system (UF water), areverse osmosis system (RO water), a water distillation system (distilledwater) and tap water. UF water and RO water are used for handwashing beforesurgery, and distilled water for the preparation of drugs. All 10 samples oftap water examined were contaminated with <10 colony forming units (cfu)/mL.Thirteen (68%) of 19 samples of UF water, nine (53%) of 17 samples of ROwater and 15 (79%) of 19 samples of distilled water were contaminated with101-104 cfu/mL. The majority of micro-organisms were non-fermentativebacteria such as Sphingomonas paucimobilis and CDC gr. IV C-2. Japanesehospitals commonly use UF water and RO water for preoperative handwashingunder the assumption that it is sterile. Our results suggest, however, thatthese types of water are inferior microbiologically to tap water. Distilledwater from the dispensary was also contaminated with micro-organisms. Theavailable chlorine content of tap water was 0·17-0·42 ppm and that of UFwater (from tap water) 0-0·06 ppm. There was no available chlorine in ROwater or distilled water (each from tap water). The reduction ordisappearance of available chlorine appears to be associated with microbialcontamination of UF water, RO water and distilled water.Copyright 1998 The Hospital Infection Society.

Of course, neither distilled water nor the RO alone can compare to an RO/DIunit at 99.9% pure. But then again, unless you also happen to havesaltwater reefs in your house, it is unlikely that you have such a setup. Ihave such a setup. The initial purchase price was ~$200. It costs onaverage ~4 cents per gallon (including original purchase price, replacementfilters, and replacement deionizing cartridges) over a 4 year lifecycle.This also produces the best drinking water available. My fish and cigarsalso like it.