The isomerization and decomposition of ClOO and OClO radicals and related Cl+O-2 and O+ClO reactions have been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the PW91PW91/6-311+G(3df ) level and their energies refined by single-point calculations with the modified Gaussian-2 method. Predicted bond-dissociation energies of ClOO and OClO, D-0(Cl-OO)=4.6 and D-0(O-ClO)=58.5 kcal/mol, agree well with experimental values. Calculated rate constants for the Cl+O-2-->ClOO reaction in 160-1000 K at the high- and low-pressure limits can be expressed by k(1)(infinity)=1.8+/-0.1x10(-10) cm(3) molecule(-1) s(-1) and k(1)(0)(He)=1.66x10(-19) T-5.34 exp(-675/T) and k(1)(0)(O-2)=1.26x10(-16) T-6.22 exp(-943/T) cm(6) molecule(-2) s(-1). For Ar and N-2, theory underpredicts k(1)(0)(M) below room temperature due to significant contributions from the "chaperon" mechanism involving Cl-M complexes. The corresponding rate constants for O+ClO-->OClO are predicted to be: k(2)(infinity)=4.33x10(-11) T-0.03 exp(43/T) cm(3) molecule(-1) s(-1) and k(2)(0)=8.60x10(-21) T-4.1 exp(-420/T) cm(6) molecule(-2) s(-1) for 200-1000 K with N-2 as the third body. The O+ClO reaction producing Cl+O-2 via ClOO was found to be pressure-independent with k(3)=4.11x10(-11) T-0.06 exp(42/T) cm(3) molecule(-1) s(-1). For the dissociation of ClOO, the rate constants are predicted to be: k(-1)(infinity)=6.17x10(15) T-0.46 exp(-2570/T) s(-1) and k(-1)(0)=1.89x10(7) T-5.88 xexp(-3280/T) cm(3) molecule(-1) s(-1) for 160-500 K with O-2 as the third-body. The corresponding rate constants for OClO dissociation can be given by: k(-2)(infinity)=1.11x10(16) T(-0.28)exp(-29600/T) s(-1) and k(-2)(0)=1.64x10(-47) T-11.0 exp(-16700/T) cm(3) molecule(-1) s(-1) for 200-2500 K with N-2 as the third body. All of the predicted rate constants, with the exception mentioned above, are in close agreement with the available experimental results. (C) 2003 American Institute of Physics.