The adsorption of HCl on the surface of H2O ice has been measured at temperatures and pressures relevant to the upper troposphere and lower stratosphere. The measured HCl surface coverage is found to be at least 100 times lower than currently assumed in models of chlorine catalyzed ozone destruction in cold regions of the upper atmosphere. Measurements were conducted in a closed system by simultaneous application of surface spectroscopy and gas phase mass spectrometry to fully characterize vapor/solid equilibrium. Surface adsorption is clearly distinguished from bulk liquid or solid phases. From 180 to 200 K, submonolayer adsorption of HCl is well described by a Bragg-Williams modified Langmuir model which includes the dissociation of HCl into H+ and Cl- ions. Furthermore, adsorption is consistent with two distinct states on the ice substrate, one in which the ions only weakly adsorb on separate sites, and another where the ions adsorb as an H+-Cl- pair on a single site with adsorption energy comparable to the bulk trihydrate. The number of substrate H2O molecules per adsorption site is also consistent with the stoichiometry of bulk hydrates under these conditions. The ionic states exist in equilibrium, and the total adsorption energy is a function of the relative population of both states. These observations and model provide a quantitative connection between the thermodynamics of the bulk and interfacial phases of HCl/H2O, and represent a consistent physicochemical model of the equilibrium system. (C) 2004 American Institute of Physics.