The reaction of HO 2 with ClO has been investigated by ab initio molecular orbital and variational transition state theory calculations. The geometric parameters of the reaction system HO 2 + ClO were optimized at the B3LYP and BH&HLYP levels of theory with the basis set 6-311+G(3df,2p). Both singlet and triplet potential energy surfaces were predicted by the modified Gaussian 2 (G2M) method. On the singlet surface, the reaction forms two HOOOCl isomers lying below the reactants by 20 kcal/mol. Their stabilization contributes significantly to the observed overall HO 2 + ClO rate constant. The predicted high- and low-pressure association rate constants for the 150-600 K range can be represented by k ∞ = 9.04 × 10 -17 T 1.22 exp(897/T) cm 3 molecule -1 s -1 and k 0 = 9.33 × 10 -24 T -3.45 exp(472/T) cm 6 molecule -2 s -1 for N 2 as the third-body. Dissociation of these excited HOOOCl intermediates produces HO + ClOO, HCl + 1 O 3 , HOCl + 3 O 2 , and HO + OClO as minor products via multistep mechanisms. On the triplet surface, formation of HOCl + 3 O 2 dominates; it occurs via a long-lived O 2 H⋯OCl complex with 3.3 kcal/mol binding energy. The complex decomposes to give the product pairs with a small (0.1 kcal/mol) barrier. The predicted rate constant can be represented by k(HOCl + 3 O 2 ) = 1.64 × 10 -10 T -0.64 exp(107/T) cm 3 molecule -1 s -1 in the temperature range 150-1000 K. The total rate constants predicted for 1-760 Torr N 2 pressure exhibit a strong negative temperature dependence below 1000 K. In the 200-400 K range, where most kinetic data have been obtained, the agreement between theory and experiment is excellent. For combustion applications, rate constants for all bimolecular product formation channels have been predicted for the 500-2500 K temperature range.