The mechanisms for ClO+NO and its reverse reactions were investigated by means of ab initio molecular orbital and statistical theory calculations. The species involved were optimized at the B3LYP/6-311 + G(3df) level, and their energies were refined at the CCSD(T)/6-311 + G(3df)//83LYP/6-311 + G(3df) level. Five isomers and the transition states among them were located. The relative stability of these isomers is ClNO2>cis-ClONO>trans-ClONO> cis-OClNO>trans-OClNO. The heats of formation of the three mosf-stable isomers were predicted using isodesmic reactions by different methods. The predicted bimolecular reaction rate constant shows that, below 100 atm, the formation of Cl+NO2 is dominant and pressure-independent. The total rate constant can be expressed as: k(ClO + NO) = 1.43 × 10 -9 T-0.83 exp(92/ T) cm3molecule -1s-1 in the temperature range of 200-1000 K, in close agreement with experimental data. For the reverse reaction, Cl+NO 2→ClNO2 and ClONO (cis and trans isomers), the sum of the predicted rate constants tor the formation of the three isomers and their relative yields also reproduce the experimental data well. The predicted total third-order rate constants in the temperature range of 200-1000 K can be represented by: k0(He)=4.89×10-16T-5.85 exp(-796/T) cm6 molecule-1 s-1 and k 0(N2) =5.72×10-15T-5.80 exp(-814/T) cm6 molecule-1 s-1. The predicted high- and low-pressure limit decomposition rates of ClNO2 in Ar in the temperature range 400-1500 K can be expressed, respectively, by: k d ∞(ClNO2) = 7.25×10 19T-1.89 exp(-16875/T) s-1 and k d 0(ClNO2)=2.51×1038T -6.8 exp(-18409/T) cm3 molecule-1 s -1. The value of kd 0(ClNO2) is also in reasonable agreement with available experimental data.