The uptake of HCl molecules by aqueous sulfuric acid droplets was measured in the temperature range 230--264 K at 39, 49, 54, 59, and 69 wt % acid and as a function of time (2--15 ms). These experiments utilized a droplet train apparatus in which a stream of monodisperse droplets (120--250 μm in diameter) is passed through a low-pressure flow containing HCl(g). The droplet area is changed in a step-wise fashion, while the HCl(g) density is continuously monitored by infrared absorption. The uptake coefficient is obtained from the measured change in the HCl density. The product of H*Dl1/2 (H*, solubility; Dl, liquid phase diffusion coefficient) and the mass accommodation coefficient α of the species as a function of temperature and sulfuric acid concentration were obtained from the uptake coefficient. The good agreement of measured and modeled H*Dl1/2 values validates current formulations of HCl reactivity in stratospheric aerosols. While the solubility of HCl decreases steeply with sulfuric acid concentration because increasing acidity reduces the dissociation of HCl into H+ and Cl- in solution, the mass accommodation coefficient is independent of acid concentration in the region studied. As with previously studied species, α is inversely proportional to temperature increasing from ~0.06 at 294 K to near unity at ~230 K. The mass accommodation coefficient is well expressed in terms of an observed Gibbs free energy as α/(1-α)=exp(-ΔGobs/RT), suggesting that the clustering model for the accommodation process is applicable in this case as well. The mass accommodation measurements are well fitted by the parameters ΔHobs=-13.8¿0.9 kcal mol-1 and ΔSobs=-52.2¿0.3 cal mol-1 K-1. Under stratospheric conditions α for HCl is unity. Implications of the HCl uptake studies for atmospheric chemistry are examined. ¿ 1998 American Geophysical Union