The mechanism for the reaction ClO+ClO3 on both singlet and triplet state potential surfaces has been investigated with the modified Gaussian-2 method based on the B3LYP/6-311+G(3df ) optimized stationary-point geometries. The result shows that the barrierless association reaction producing ClOClO3 and two lower barrier O-atom abstraction reactions take place primarily on the singlet state potential surface; they are energetically more favorable than those occurring on the triplet state surface. Rate constants calculated by variational transition state and Rice-Ramsperger-Kassel-Marcus theories suggest that the major products are ClOClO3 at low temperatures (<700 K) and OClO+ClOO at high temperatures. The following rate constants are recommended for atmospheric chemistry and combustion applications: k(1)(infinity)(ClOClO3)=1.43x10(-10) T(0.094)exp(-82/T) cm(3) molecule(-1) s(-1) (200-3000 K) and k(1)(0)(ClOClO3)=0.14 T(-10.19)exp(-1597/T) cm(6) molecule(-2) s(-1) (200-800 K) for He as the third-body. In addition, a potential high-temperature reverse reaction involving Cl and ClO4 has been predicted to yield exclusively the ClO and ClO3 products; its rate constant is predicted to be: k(4)(ClO+ClO3)=8.05x10(-11) T-0.158 exp(-49/T) cm(3) molecule(-1) s(-1) (200-3000 K). The heats of formation at 0 K for s-ClO3, ClO4, and ClOClO3 have been predicted to be 46.7, 59.4, and 38.3 kcal/mol with about 1 kcal/mol uncertainty using the new heat of formation of OClO, 24.1+/-0.1 kcal/mol, based on the most recent bond dissociation energy of O-ClO reported in the literature. (C) 2003 American Institute of Physics.