Association of alkyl radicals with ground-state oxygen 31, (03) generates chemically activated peroxy intermediates, which can isomerize or further react to form new products before collisional stabilization. The lowest-energy reaction (similar to 19 kcal mol(-1)) for alkylperoxy derivatives of C-3 and larger n-hydrocarbons is an isomerization (intramolecular H-atom transfer) that forms a hydroperoxide alkyl radical, and there is a 30 kcal mol(-1) barrier path to olefin plus HO2, which is a termination step at lower temperatures. The low-energy-barrier product, hydroperoxide alkyl radical intermediate, can experience additional chemical activation via association with a second oxygen molecule, where there are three important paths that result in chain branching. The competition between this HO2 + olefin termination step of the first 0, association and the chain branching processes from the second chemical activation step plays a dominant role at temperatures below 1000 K. Secondary n-pentyl radicals are used in this study as surrogates to analyze the thermochemistry and detailed kinetics of the chemical activation and stabilized adduct reactions important to chain branching and termination. As these radicals provide six- member ring transition states for H-atom transfer between secondary carbons, they represent the detailed kinetics of larger alkane radicals, such as the common fuel components n-heptane and n-decane. Comprehensive potential energy diagrams developed from multilevel CBS-QB3, G3MP2, and CBS-APNO and single-level ab initio and density functional theory methods are used to analyze secondary 2-pentyl (n-pentan-2-yl) and interrelated 2-hydroperoxide-pentan-4-yl radical interactions with O-2. The thermochemistry and kinetics of the chemical activation and stabilized adduct reactions important to chain branching and termination are reported and discussed. Results show that the chain branching reactions have faster kinetics in this system because the barriers are lower than those observed in ethyl and propyl radical plus 07 reactions; consequently, the branching is predicted to be more important. The lower barriers for branching result in less competition from the termination (HO2 + olefin) path in this larger radical. Several nontraditional reaction channels not previously considered in the literature are identified. A pathway is suggested to explain the formation of a unique trioxane product observed experimentally.