The nature of the interaction of HCl vapor with ice has been investigated, using thermal analysis and FTIR spectroscopy to characterize the ice substrate, and mass spectrometry to measure the concentration of HCl and H2O vapors in the gas phase. The results indicate that a liquid layer is formed rapidly at the ice surface for ice exposed to HCl vapor at partial pressure above those characteristic of the ice--liquid (aqueous HCl solution) equilibrium system. This liquid layer also forms below the eutectic temperature (186 K); that is, it forms even at temperatures at which the liquid is metastable with respect to the formation of HCl trihydrate. For smaller HCl partial pressures such as those prevailing in the stratosphere, the HCl is taken up by the ice surface in amounts corresponding to a large fraction of a monolayer. The chemical reactivity of this surface HCl is very large; Chlorine activation by type II polar stratospheric clouds (consisting of ice particles) should occur efficiently by reaction of the HCl with ClONO2. ¿ American Geophysical Union 1992