The gas-phase redox reactions of HOCl with its self and HNO x(x =1-3) have been studied theoretically by ab initio quantum chemical and statistical mechanical theories. The structures of reactants, intermediate complexes, products, and transition states were optimized at the MPW1PW91/6-311+G(3df, 2p) level of theoiy. The potential energy surface of each reaction was refined at the CCSD(T)/6-311+G(3df,2p) level of theory. The most favorable products are predicted to be ClClO + H 2O and ClOCl + H 2O for the HOCl self-reaction (A), H 2O + Cl + NO for the HOCl + HNO reaction (B), H 2O + ClNO 2 for the HOCl + HONO-t reaction (C), H 2O + cis-ClONO for the HOCl + HONO-c reaction (D). For the HOCl + HONO 2 reaction (E), only one dehydration reaction channel was considered to produce H 2O + ClONO 2. The rate constants of all above five reactions have been predicted at 300-3000 K by the VTST/RRKM theory. The calculation shows that the theoretical rate constants are within the upper limits of experimental results. In addition, we calculated the equilibrium constant for the Cl 2O + H 2O -rarr; HOCl + HOCl reaction, which is also in reasonable agreement with experimental data within the error of the available experimental enthalpy change.