Six reaction systems relevant to the atmosphere and the ammonium perchlorate combustion chemistry have been studied. The systems include the bimolecular reactions of ClO with HO, HO2, ClO, OClO, NO and NO2 and some of their reverse processes. The geometries of the reactants, intermediates, transition states and products involved in these reactions were optimized at the B3LYP/6-311+G(3df, 2p) level and nal energies were rened at the modied Gaussian - 2 (G2M) or CCSD(T)/6-311+G(3df, 2p) level. Temperature/pressure-dependent rate constants for major product channels have been predicted and compared with the available experimental data. Detailed mechanisms for these reactions have been discussed on the basis of predicted potential energy surfaces. The calculated geometrical parameters, heats of formation and rate constants are quite satisfactory and in good agreement with available experimental data. For practical applications, the heats of formation of the key species, and the rate constants for all the major reactions discussed under different conditions have been recommended.