Alkylperoxy radicals, produced by the photolysis of solutions of di-t-butyl peroxide in oxygen-saturated alkanes, have been detected by electron spin resonance spectroscopy. Tertiary alkylperoxy radicals terminate with second-order kinetics, and absolute termination rate constants have been measured over a range of temperatures (–60 to +20°C). The process has an unusually high activation energy (ca. 8 kcal mol^–1) for a reaction involving two radicals. Secondary alkylperoxy radicals decay more rapidly by a reaction which has a smaller activation energy (ca. 2 kcal mol^–1). The decay of primary radicals is so rapid that the radical concentration is too small to be detected. A new method is described for the measurement of cross-termination rate constants.