Photolytically generated alkylperoxy radicals have been detected and their concentration measured by electron spin resonance spectroscopy. The rate of the irreversible decay of tertiary alkylperoxy radicals is negligible below –115°C. However, within the temperature range –160 to –115°C, a reduction in temperature causes the concentration of radicals to decrease to a new value which, once achieved does not vary with time. The process is reversible. This behaviour is attributed to the existence of an equilibrium between two alkylperoxy radicals and a tetroxide molecule. Average values for the associated changes in entropy ΔS° and enthalpy ΔH° are –38 ± 4 cal deg^–1 mol^–1 and –8.9 ± 0.5 kcal mol^–1 respectively. These thermodynamic constants have been combined with the rates of the irreversible decay of alkylperoxy radicals, measured at higher temperatures (–60 to +20°C) to provide a more complete description of the termination process.