It has been suggested that some Type I polar stratospheric clouds (PSCs) are supercooled liquids containing sulfuric acid, water, and nitric acid. We show that the activation of chlorine over supercooled liquids is comparable to that over solid Type I PSCs at the same temperature. Furthermore, at temperatures within a few degrees of the frost point, the reactions of ClONO2 with HCl and H2O, that of HOCl with HCl, and the hydrolysis of N2O5 would be faster if Type I PSCs are liquid solutions rather than solids. Thus chlorine can be rapidly activated without the formation of solid Type I PSCs. ¿ American Geophysical Union 1996