The reaction probabilities of N2O5 and ClONO2 with H2O on liquid sulfuric acid surfaces have been reexamined to determine the effect of HNO3 on these two hydrolysis rates, using a fast flow reactor coupled to a chemical ionization mass spectrometer. The measurements were carried out by maintaining constant H2O and HNO3 partial pressures and by varying temperatures between 227 and 195 K in order to mimic compositions representative of stratospheric aerosols. For experiments excluding HNO3, the reaction probability of N2O5 hydrolysis was found to be near 0.1, independent of temperature and H2SO4 content. This is in agreement with results previously measured under similar conditions. In the presence of gaseous HNO3 at stratospheric concentrations, the reaction probability was observed to decrease from about 0.09 at 218 K to about 0.02–0.03 at 195 K for PH2O = 3.8 × 10−4−1.0 × 10−3 Torr, showing that incorporation of HNO3 into liquid sulfuric acid greatly retarded the N2O5 hydrolysis. The ClONO2 reaction with H2O on liquid sulfuric acid, on the other hand, did not appear to be affected by the presence of HNO3. Copyright 1995 by the American Geophysical Union.