The reaction of ClONO2 with H2O on various water clusters has been investigated via an ab initio calculation at the MP2//HF/6-31G(d) level. The calculations have shown that as more water molecules are involved in the reaction, the barrier height: drops dramatically. The barrier energies of the reaction on the water clusters vary with the extent of hydration. A value as low as 3.2 kcal/mol was observed in the calculation. It is suggested that the ice surface shows catalytic character for the heterogeneous reaction through structure catalysis and hydration. It is shown that our mechanism of ClONO2 hydrolysis on ice surfaces is in a broad sense compatible with the ion-catalyzed mechanism.