The mechanism for the reaction ClO+OClO has been investigated by ab initio molecular orbital and transition state theory calculations. Nine stable isomers of Cl2O3 (including optical isomers) are located at the B3LYP/6-311+G(3df ) level. The transition states between pairs of isomers are explored and the stability of the isomers and their dissociation mechanisms are discussed. The relative stability predicted by the modified Gaussian-2 (G2M) method at the G2M//B3LYP/6-311+G(3df ) level is ClOCl(O)O>ClOOOCl(C-2)>ClOOOCl(C-s)>ClClO3(C-3v)>cyc-ClOOCl(O)>ClOOClO. The formation of ClOCl(O)O is dominant at low temperatures, taking place barrierlessly with the second and third order rate constant, k(1)(infinity)=3.0x10(-10) cm(3) molecule(-1) s(-1) and k(1)(0)=1.1x10(-17) T-5.5 exp(-398/T) cm(6) molecule(-2) s(-1) in the temperature range of 200-400 K for N-2 as the third-body. Over 500 K, formation of ClOO+ClO becomes dominant and ClOCl+O-1(2) is also competitive. Their overall rate constants can be given by k(ClOO)=1.0x10(-22) T-2.8 exp(-78/T) and k(ClOCl)=9.6x10(-22) T-2.4 exp(-1670/T)cm(3) molecule(-1) s(-1), respectively, in the temperature range of 500-2500 K. (C) 2003 American Institute of Physics.