A Knudsen cell reactor equipped with FTIR reflection-absorption spectroscopy (FTIR-RAS) has been used to study the hydrolysis of chlorine nitrate (ClONO2) on thin nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD) films. The reaction efficiencies and condensed-phase products have been measured for a variety of water partial pressures at 185 K. The reaction efficiencies for ClONO2 hydrolysis on NAT and NAD were of similar magnitude and increased from gamma = 0.0004 to gamma = 0.007 as the water partial pressure was increased from 5 x 10(-6) to 1.4 x 10(-4) Torr. Although these values agree well with literature values at 90% relative humidity, significant differences are observed at lower relative humidities. Using Our results at 185 K along with previous studies at 191 and 202 K, we suggest that this reaction may have a temperature dependence that is not currently addressed in atmospheric models. In the condensed phase, we observed the water dependence by the formation of progressively water-rich surface layers. The amount of water incorporated onto the NAT and NAD films increased with increasing relative humidity, suggesting that ClONO2 hydrolysis occurs most efficiently on a water-rich hydrate surface. For water saturation ratios S greater than or equal to 1.5, nucleation and growth of crystalline ice was observed to occur in the aqueous HNO3 surface layer on top of the reactant NAD and NAT films.