The direct ClONO2 + HCl --> Cl-2 + HNO3 reaction on ice, implicated in polar stratospheric ozone depletion, is studied quantum chemically on a model ice lattice comprising nine water molecules. The reaction path is calculated at the HF/(HW*,3-21G) level, using the Hay-Wadt effective core potential for Cl. At these geometries, energies are recalculated at the MP2/(SBK+*,6-31+G*) level, with the Stevens-Bash-Krauss effective core potential for Cl. HCl is found to be ionized in the reactant complex. The calculated reaction internal energy barrier, including zero-point energy correction, is 6.4 kcal/mol. The reaction mechanism involves proton transfer in the ice lattice, accompanied by nucleophilic attack of Cl- on the Cldelta+ in ClONO2; the lattice is an active participant in the reaction. Implications for heterogeneous atmospheric chemistry are discussed.