Kinetic information is presented on the decomposition of cumene hydroperoxide at 110 " C and 60-380 bar in supercritical krypton, xenon, carbon dioxide, propane, and chlorodifluoromethane and a t ambient pressure in liquid octane, l-octene, l-hexanol, and cyclohexanol. The supercritical reaction studies are performed with a high-pressure reactor constructed of 316 stainless steel (316SS), gold-plated 316SS, and Teflon-coated 31621s. The magnitude of the reaction rate constant is a strong function of the metals present during the reaction and of the ability of the solvent to hydrogen bond or complex with the activated complex of cumene hydroperoxide, and it is a weak function of hydrostatic pressure and solution viscosity over the pressure ranges investigated. Gold, 316SS, and aluminum facilitate the formation of geminate radicals, while gold also affects the selectivity of the reaction. Transition-state theory is used to interpret the reaction data.