Experimental measurements of the heterogeneous reactivity of sulfuric acid-water mixtures at low temperatures are presented. The kinetic data are combined with a stratospheric aeroso-loading model and simple chemical models to predict the effect of heterogeneous chlorine and chlorine oxide radical reactions on the stratospheric Cl/HCl ratio at several altitudes. The Cl reactions are found to have little influence, but the ClO reaction reduces the Cl/HCl ratio by about 3% at 20 km (within a factor of 2 at 1 standard deviation). The effect at higher altitudes is smaller. Simulated sunlight was found to have no effect on the heterogeneous reactivities.