Chlorine nitrate adsorption kinetics and uptake have been measured on ordered ice and HCl trihydrate films at temperatures below 150 K. Reaction was followed using a thermal molecular beam, with mass spectrometric detection of gas-phase products and temperature-programmed desorption (TPD) and IR to identify adsorbed species. The sticking probability (S) on pure water ice is (0.98 +/- 0.03) at 85 K and remains near unity for temperatures up to 145 K. Initially S is independent of the ClONO2 uptake, indicating a trapping mechanism for reaction. A molecular state which desorbs near 120 K during TPD is identified as a precursor state and above this temperature molecular ClONO2 is not stable on the surface and adsorption forms HOCl and nitric acid hydrate. On a clean ice surface the reaction probability starts to decrease after 0.1 monolayer of ClONO2 has adsorbed, reaction ceasing (S < 5 x 10(-2)) at an uptake of (0.25 +/- 0.05) monolayer, independent of temperature. Reaction occurs even at low temperature, where the surface is immobile, indicating that ClONO2 hydration can occur on the ice surface and does not require an extensive hydrate cage. The saturation stoichiometry is consistent with a surface covered with HOCl and an amorphous nitric acid trihydrate film. The initial reaction probability for ClONO2 Uptake onto pure HCl trihydrate films was similar, (0.98 ::L 0.05) for temperatures between 85 and 145 K with some chlorine desorbing promptly into the gas phase even at 85 K. The uptake of ClONO2 increases from 0.5 to ca. I monolayer above 125 K, the temperature at which HCl starts to transport into water ice films, reaction extending beyond the top layer of the HCl trilrydrate surface.