Thermodynamic properties of solid phases containing H2O and HCl are examined for conditions expected over Antarctica in winter. It is shown that solid solutions with 0.02--0.035 mole fraction HCl in H2O ice will form in the stratosphere at temperatures between 193¿ and 190 ¿K. Crystalline HNO3⋅H2O and/or HNO33H2O form in the same temperature range. Thus condensation of small quantities of H2O (<1 ppm) leads to nearly complete removal of both HCl and HNO3 from the gas phase. Formation of ice-HCl solid solutions provides a favorable setting for the heterogeneous reaction of HCl with ClNO3, initiating a rapid sequence of reactions that converts solid-phase HCl into gaseous ClNO3 or chlorine oxide radicals. Odd nitrogen species are simultaneously converted into solid-phase nitrates. The cycle producing reactive chlorine gases is likely to be completed before particles grow large enough to fall from the stratosphere. Hence precipitation of particles from the polar stratospheric clouds is expected to remove odd nitrogen from the stratosphere efficiently, leaving chlorine gases behind. High concentrations of reactive chlorine oxide radicals are rapidly produced by heterogeneous reactions if the initial concentration of HCl exceeds a critical value, 0.5 times the concentration of NOx before onset of condensation. Production of unreactive ClNO3 and HOCl is favored if HCl levels are lower than this value. The onset of Antarctic ozone depletion in the late 1970s may in part reflect growth of HCl levels beyond this threshold. ¿ American Geophysical Union 1988