In an attempt to understand the mechanism of the reaction of alkylperoxy radicals with hydroperoxy radical, a key reaction in both atmospheric and combustion chemistry, the singlet and triplet potential energy surfaces (PESs) for the gas-phase reaction between CH3O2 center dot and HO2 center dot leading to the formation of CH3OOH and O-2 have been investigated by means of quantum-mechanical electronic structure methods (CASSCF and CASPT2). In addition, standard transition state theory calculations have been carried out with the main purpose of a qualitative description of the strong negative temperature dependence observed for this reaction. All the pathways on both the singlet and triplet PESs consist of a reversible first step involving the barrierless formation of a hydrogen-bonded prereactive complex, followed by the irreversible formation of products. This complex is a diradical species where the two unpaired electrons are not used for bonding and is lying about 5 kcal/mol below the energy of the reactants at 0 K. The lowest energy reaction pathway occurs on the triplet PES and involves the direct H-atom transfer from HO2 to CH3O2 in the diradical complex through a transition structure lying 3.8 kcal/mol below the energy of the reactants at 0 K. Contradicting the currently accepted interpretation of the reaction mechanism, the observed strong negative temperature dependence of the rate constant is due to the formation of the hydrogen-bonded diradical complex rather than a short-lived tetraoxide intermediate CH3OOOOH.