The heterogeneous surface reaction of OH with dry KI(100) results in iodide vacancies in the surface lattice sites that are filled with OH to generate a stable layer of KOH. Under high-vacuum conditions, in which surface ions are not mobile, the reaction is self-passivating and generates two molecular layers of potassium hydroxide, releasing 1.6 x 10(16) iodide ions per cm(2) of surface area. Reaction rates are identical with those of NaI(100). A similar surface reaction occurs with alkali bromides (KBr(100)), albeit at a much slower rate to generate approximately one-tenth of a monolayer of KOH, whereas no observable reaction occurs with KCl(100) under the conditions of this experiment. The heterogeneous reaction of OH with alkali halides is found to be dependent solely on the identity of the halide anion and independent of the alkali metal cation with the relative reaction rates following the anion ordering, I- > Br- > Cl-. The release of halide-containing species is expected to impact the chemistry of the marine boundary layer.