Hydrated chlorine nitrate and hydrogen chloride ClONO2 center dot HCl center dot(H2O)(n) (n = 0-3) clusters were investigated by using the MP2/aug-cc-pVTZ level of theory to clarify the reaction mechanism of Cl-2 production. Isomeric stable structures found in n = 2 and 3 clusters have equivalent binding energies and the reaction barrier drastically decreases to be 2.1 kcal/mol at n = 3. The plausible reaction pathways were proposed according to calculated structures and energies, where the zero-point-energy corrections are important to determine the energy profiles of reactions especially for the n = 3 cluster. The kinetic analysis using the transition state theory suggested that the reaction rate constant from the original reactants to the product of n = 3 is 1.8 x 10(5) times larger than that of n = 2 cluster. Even though the small amount of the molar concentration of HCl(H2O)(3) is obtained, the overall reaction rate of the trihydrate reaction is still 35 times faster than that of the dihydrate.