The kinetics and mechanism for the reaction of ClOO with NO have been investigated by ab initio molecular orbital theory calculations based on the CCSD(T)6-311G(3df)PW91PW916-311G(3df) method, employed to evaluate the energetics for the construction of potential energy surfaces and prediction of reaction rate constants. The results show that the reaction can produce two key low energy products ClNO 3O 2 via the direct triplet abstraction path and ClO NO 2 via the association and decomposition mechanism through long-lived singlet pc-ClOONO and ClONO 2 intermediates. The yield of ClNO O 2 ( 1) from any of the singlet intermediates was found to be negligible because of their high barriers and tight transition states. As both key reactions initially occur barrierlessly, their rate constants were evaluated with a canonical variational approach in our transition state theory and Rice-Ramspergen-Kassel-Marcus/master equation calculations. The rate constants for ClNO 3O 2 and ClO NO 2 production from ClOO NO can be given by 2.66 × 10 -16 T 1.91 exp(341T) (200-700 K) and 1.48 × 10 -24 T 3.99 exp(1711T) (200-600 K), respectively, independent of pressure below atmospheric pressure. The predicted total rate constant and the yields of ClNO and NO 2 in the temperature range of 200-700 K at 10-760 Torr pressure are in close agreement with available experimental results.