High-level electronic structure calculations were used to study the mechanism of the reaction of ClONO2 with HCl in neutral water clusters containing one to five solvating water molecules. For the reaction between molecular HCl and ClONO2, the barrier decreases from 42 kcal mol(-1) (uncatalyzed) to essentially zero when catalyzed by only two water molecules, where the reaction products involve Cl-2 and HONO2. The calculations thus predict that the gas-phase reaction may be important in the stratospheric reactivation of ClONO2. The reaction between ClONO2 and solvated H3O+Cl-, as on the polar stratospheric cloud (PSC) surface, was investigated with clusters involving up to seven water molecules. The ice-catalyzed reaction involves an ionic mechanism whereby charge transfer to ClONO2 from the attacking nucleophile leads to significant ionization along the Cl-ONO2 bond. The effect of the size of the first solvation shell of Cl- is addressed by our calculations. In a cluster containing three waters and a five-water cluster structurally related to hexagonal ice, ClONO2 reacts spontaneously with HCl to yield Cl-2/HONO2 in the three-water reaction and Cl-2/H3O+-NO3- in the five-water-catalyzed reaction. The calculations thus predict that the reaction of ClONO2 with HCl on PSC ice aerosols can proceed spontaneously via an ionic pathway.