The heterogeneous interactions of ClONO2, HCl and HNO3 with sulfuric acid surfaces were studied using a Knudsen cell flow reactor. The surfaces studied, chosen to simulate global stratospheric particulate, were composed of 65--75% H2SO4 solutions at temperatures in the range --63 to --43¿C. Heterogeneous loss, but not reaction, of HNO3 and HCL occurred on these surfaces and the measured sticking coefficients are reported. Chlorine nitrate reacted on the cold sulfuric acid surfaces, producing gas-phase HOCL and condensed HNO3. ClONO2 solution at --63¿C, forming gaseous Cl2. In all cases studied, the sticking and/or reaction coefficients were much larger for the 65% H2SO4 solution at --63¿C than for the 75% solution at --43¿C. ¿ American Geophysical Union 1988