With the objective of gaining insight into how heterogeneous reactions occur in the Antarctic stratosphere, we have initiated a program to study the adsorption and reaction of simple molecules on model polar stratospheric cloud surfaces. In this work, the temperature-programmed desorption of hydrogen chloride from ultrathin (5-20 monolayers thick) water films is described. Two distinct HCl desorption states, designated alpha- and beta-HCl, are observed at 140 and 180 K, respectively. Water sublimation occurs at 180 K and is concurrent with beta-HCl evolution. beta-HCl, which is formed exclusively at low HCl exposures, is derived from the thin film bulk, while alpha-HCl is associated with an adsorbed state. beta-HCl is assigned to the sublimation of a stoichiometric phase of HCl and water, probably HCl.6H(2)O, and alpha-HCl is assigned to the thermal desorption of HCl from the hexahydrate surface. Desorption spectra of HCl from ice-d(2) show that H-D exchange between HCl and D2O is much less than would be expected for a dissociatively adsorbed state of HCl. The cu state is therefore assigned to molecularly adsorbed HCl. The activation energy for alpha-HCl desorption is 33+/-5 kJ.mol(-1), a value which is highly suggestive of formation of a hydrogen bond between HCl and the hexahydrate surface. Two possible structures of HCl adsorbed on the hexahydrate surface are considered. Implications of these results for heterogeneous polar stratospheric chemistry are discussed.