The reaction of ClO with Cl and its related reverse processes have been studied theoretically by ab initio quantum chemical and statistical mechanical calculations. The geometric parameters of the reactants, products, and transition states are optimized by both UMPW1PW91 and unrestricted coupled-cluster single and double excitation (UCCSD) methods with the 6-311+G(3df) basis set. The potential energy surface has been further refined (with triple excitations, T) at the UCCSD(T)/6-311+G(3df) level of theory. The results show that Cl2 and O (3P) can be produced by chlorine atom abstraction via a tight transition state, while ClOCl ( 1A1) and ClClO (1A′) can be formed by barrierless association processes with exothermicities of 31.8 and 16.0 kcal/mol, respectively. In principle the O (1D) atom can be generated with a large endothermicity of 56.9 kcal/mol; on the other hand, its barrierless reaction with Cl2 can readily form ClClO ( 1A′), which fragments rapidly to give ClO + Cl. The rate constants of both forward and reverse processes have been predicted at 150-2000 K by the microcanonical variational transition state theory (VTST)/Rice- Ramsperger-Kassel-Marcus (RRKM) theory. The predicted rate constants are in good agreement with available experimental data within reported errors.