The reaction of ClONO2 with HCl on the nitric acid trihydrate (NAT) and dihydrate (NAD) and on frozen sulfuric acid solutions was studied as functions of reactant concentration and temperature representative of the stratosphere. A flow tube reactor equipped with a chemical ionization mass spectrometer detector was used. The measured reactive uptake coefficient shows a strong dependence on temperature at constant H2O partial pressure. Our results for NAT are qualitatively similar to but quantitatively different from those of Abbatt and Molina <1992a>. The hydrolysis of N2O5 and ClONO2 on frozen sulfuric acid were measured to be far less efficient than that in liquid solutions. The implications of these results to the heterogeneous chemistry of the stratosphere are discussed. ¿ American Geophysical Union 1993