Existing laboratory data for reaction rates on ice and nitric acid trihydrate are interpreted in terms of a simple, speculative mechanism for these reactions. The mechanism assumes that the solid phase is either pure H2O or pure HNO3(H2O)3 and that the state of the surface depends largely on the gas phase composition. This dependence is described by Langmuir adsorption isotherms. The adsorption of a non-reactive species can affect the rate of reaction by inhibiting the adsorption of reactive species. In particular, differences in reaction rates observed on ice and HNO3(H2O)3 are attributed to the adsorption of gas phase HNO3 produced by the decomposition of HNO3(H2O)3. The interpretation of heterogeneous reaction rates requires detailed measurements of the rates as a function of gas phase composition. ¿ American Geophysical Union 1993