We present a model of heterogeneous reactivity based on physical adsorption that describes the observed relative humidity dependence of the ClONO2 reaction probability with H2O and HCl on sulfuric acid tetrahydrate ice surfaces (SAT) and with H2O on nitric acid trihydrate (NAT). The laboratory data are modeled using only two parameters for a given system, the measured reaction probability on a neat H2O ice surface, and a constant from the BET theory which describes the fraction of an acid hydrate surface covered by H2O as a function of relative humidity. The model indicates that ClONO2 reactivity with both HCl and H2O on SAT and with H2O on NAT is controlled by the surface coverage of H2O. In contrast, the reaction of ClONO2+HCl on NAT is better described by an alternative model based on reactivity in solutions formed within a porous ice by capillary liquid absorption. ¿ American Geophysical Union 1996