The reaction of iodine, a potential disinfectant for use in the treatment of recycled water during long-duration manned space missions, and several organic substrates that are expected chemical constituents in a closed-loop recycle water system, yields iodinated disinfection by-products. The reactions were studied using procedures analagous to those developed by the U. S. Environmental Protection Agency for evaluation of chlorinated disinfection by-products in water. The iodinated products formed in these studies were identified using gas chromatography with both electron capture and mass spectrometric detection. Aqueous solutions of acetic acid and of dextran produce iodinated alkyl-compounds when treated with iodine, as triiodide ion, at neutral pH. Similar treatment of phenol yields iodine-substituted phenols at appreciable concentrations.