The uptake of HCl molecules by aqueous sulfuric acid droplets was measured at 283 K as a function of acid mole fraction. In the apparatus a controllable stream of monodispersed droplets is produced by a vibrating orifice jet. The droplets enter a flow system containing HCl(g). The droplets are switched on and off while the density of the species is continuously monitored by infrared absorption. From the on-off signal a coefficient for the probability of gas uptake collision with the liquid surface is calculated. This coefficient is 0.15 for pure water, remains constant up to 0.1 mole fraction of H2SO4 (40 wt %) and then drops sharply to <10-2 at 0.2 mole fraction (60 wt %). The results are in accord with calculations based on the decrease in the solubility of HCl with increasing H2SO4 mole fraction. Increasing acidity reduces the dissociation of HCl into H+ and Cl- in solution. Our results indicate that mass accommodation coefficients are large enough so that the HCl content of stratospheric aerosols will be determined entirely by this solubility. ¿ American Geophysical Union 1990