The heterogeneous reaction HOCl+HCl→Cl2(g)+H2O(s) has been observed to proceed readily on cold water-ice and nitric acid trihydrate (NAT) surfaces. For reactant partial pressures in the 10-7--10-5 Torr range, the reaction probability (&ggr;) on water-ice is found to be 0.16¿0.10 at 202 K, and 0.24 +0.5/-0.15 at 195 K, as measured by the decay of HOCl vapor when the HCl partial pressure is above that of HOCl. On NAT, &ggr; is found to have a strong dependence on the H2O vapor pressure of the NAT crystals: at 202 K, reaction probabilities of 0.17 are measured for H2O vapor pressures approaching those of ice (H2O-rich NAT), whereas &ggr; drops to 0.002 if the H2O pressure is lower by a factor of five (HNO3-rich NAT). The yield of Cl2(g) relative to HOCl(g) loss is measured to be 0.87¿0.20. These results suggest that the heterogeneous process ClONO2+HCl→Cl2(g)+HNO3(s), which occurs on polar stratospheric cloud surfaces, may be comprised of two steps: ClONO2+H2O(s)→HOCl+HNO3(s), followed by HOCl+HCl→Cl2(g)+H2O(s). Also, the HOCl+HCl reaction may be directly of importance in the polar stratosphere providing an additional path for chlorine activation. ¿ American Geophysical Union 1992