A droplet train apparatus has been used to measure the heterogeneous reactive uptake of gaseous N2O5 and ClONO2 by concentrated sulfuric acid solutions. H2SO4 concentrations in the range of 39 to 69 wt % were investigated between 229 and 260 K. Uptake rates normalized to the gas-liquid collision frequency, &ggr;0, for N2O5 ranged from 0.086 to 0.16, decreasing moderately with increasing temperature and decreasing H2SO4 concentration. Uptake rates for ClONO2, measured over a slightly narrower concentration range of 39--59 wt % H2SO4, ranged between 0.0037 and 0.056, decreasing moderately with H2SO4 temperature but significantly with increasing concentration. Results are compared with measurements from other laboratories using different experimental techniques. In general, the data from the different groups agree well. A phenomenological model is presented which addresses the solubility, diffusion, and chemical reactivity of XNO3 (X=Cl, NO2) in sulfuric acid solutions and accounts for the dependence of the observed uptake rates on H2SO4 concentration and temperature. Two XNO3 hydrolysis pathways are proposed, one involving direct reaction with H2O and the other involving participation of H+ ions to promote bond dissociation. Differences between the concentration dependencies of &ggr;0 for ClONO2 and N2O5 can be ascribed largely to different rates of acid-catalyzed hydrolysis. The implications of these results for the effects of lower stratospheric sulfuric acid aerosols on ozone depletion chemistry are discussed.¿ 1997 American Geophysical Union