High-level electronic structure calculations have been used to study the mechanism of the hydrolysis of bromine nitrate in neutral water clusters containing one to eight solvating water molecules. The calculations clarify some of the uncertainties in the mechanism of halogen nitrate hydrolysis on PSC ice aerosols. The free energy barrier decreases from 42.4 kcal mol(-1) (single water) to essentially zero when catalyzed by six water molecules. As the size of the water cluster is increased, BrONO2 shows increasing ionization along the Br-ONO2 bond, consistent with the proposed predissociation in which the electrophilicity of the bromine atom is enhanced, thus making it more susceptible to nucleophilic attack from a surface water molecule. A species akin to the experimentally proposed intermediate, H2OBr+NO3-, is found to be stable in clusters containing both three and six water molecules, where the ion pair is separated by single and double layers of water molecules, respectively. For a cluster containing six water molecules, which has a structure related to that of ordinary hexagonal ice, BrONO2 is hydrolyzed to yield HOBr and ionized nitric acid (H3O+NO3-. The calculations thus predict an ionic mechanism for the hydrolysis of halogen nitrates on PSC ice aerosols.