Type I polar stratospheric clouds (PSC I) play an important role in ozone depletion reactions, and their composition may determine a key step in the heterogeneous chemistry. Formation of PSC I has been simulated by vapor depositing nitric acid and water on different substrates under conditions designed to approach those found in the polar stratosphere. Composition measurements of the resulting solids suggest the existence of a previously unreported hydrate of nitric acid, the pentahydrate. When PSC I particles condense from the vapor phase, the pentahydrate may be the nitric acid solid whose formation is favored under stratospheric temperature and pressure conditions.