Cluster chemistry employing a fast flow reactor was used to evaluate the enhanced reactivity of chlorine nitrate (ClONO2) with dissolved acid halide. Reactions of ClONO2 with deuterium chloride doped deuterated water clusters [D+(D2O)(m)(DCl)(n), (m 10-14, n = 1,2)] were investigated under thermal conditions over a temperature range of 148-158 K; deuterated species were employed in place of light hydrogen containing molecules to facilitate the determination of reaction mechanisms. Chemical reactions were found to occur between water clusters and chlorine nitrate only under conditions where dissolved deuterium chloride was present. Nitric acid is found to remain within the water cluster to yield gaseous chlorine (Cl-2) from the water cluster. These results provide direct evidence that an ionic reaction mechanism is involved in the heterogeneous reaction of hydrogen chloride and chlorine nitrate on clusters that mimic polar stratospheric cloud surfaces.