A wetted-wall flow tube was used to measure the reaction probabilities, &ggr;, of ClONO2 and N2O5 on sulfuric acid surfaces. The reaction probabilities were determined from first-order wall loss rate coefficients measured over sulfuric acid slowly flowing down the inside wall of the flow tube. Sulfuric acid content was varied from 40 to 75% by weight, and measurements were conducted at temperatures characteristic of the stratosphere. We determined the reaction probabilities of N2O5 to be 0.12¿0.03 over 40% acid at 215 K, 0.14¿0.03 over 60% acid at 215 K and 0.10¿0.02 over 70 and 75% acid at 220 and 230 K, respectively. The &ggr; for ClONO2 increased from 2¿10-4 over 75% acid at 230 K to 6.4¿10-2 over 40% acid at 215 K; these are a factor of 2 lower than a previous determination performed under the same conditions. When we introduced HCl into the flow tube, we detected negligible uptake over the 65 and 70% acid solutions, &ggr;<3¿10-5, and a small uptake, &ggr;=4¿10-4, for HCl onto 60% acid at 215 K. We did not detect a significant enhancement in the first-order loss rate coefficients for ClONO2 over these solutions in the presence of HCl. Thus the reaction ClONO2+HCl occurring on 40-75 wt. % sulfuric acid solutions will not be a significant source of activated chlorine in the stratosphere; however, loss of N2O5 and ClONO2 could affect the NOx and ClOx concentrations. ¿American Geophysical Union 1991