The solubility of HCl in cold H2SO4/H2O solutions is central to an assessment of the role of stratospheric aerosol in polar ozone destruction. The interpretation of laboratory measurements of HCl uptake by aqueous H2SO4 requires a knowledge of (1) HCl solubility and (2) the HCl liquid phase diffusion coefficient. Neither quantity is easily determined because of the high N2SO4 concentration and low temperature of the solutions, making experimental data difficult to evaluate and to apply to the real stratospheric situation. To avoid these difficulties, we have used a new thermodynamic model for an improved estimate of HCl solubility, and a viscosity-based cubic cell model to obtain the diffusion coefficient. The calculated HCl solubility in aqueous H2SO4 below 200 K is up to three orders of magnitude greater than previously thought. This might promote chlorine activation particularly during volcanically perturbed periods with an enhanced stratospheric aerosol loading. ¿ American Geophysical Union 1994