in aqueous solution and in the gas phase has opposite signs. Values for the KIE and k 6 /k 5 decrease with increasing temperature from 5 to 55°C in the gas phase, while these values increase in solution. We propose that this phenomenon is a consequence of a solvent cage effect.The reactions of hydroxyl radicals with hydrocarbons play an important role in biochemical processes and the chemistry of the atmosphere and natural water supplies [1][2][3][4]. The kinetics of the reactions of OH radicals with saturated hydrocarbons (RH)including cycloalkanes, has been studied extensively in the gas phase [1]. The absolute rate constants for reaction (1) have been measured in aqueous solution only for methane [5,6], while relative rate constants in the methane series have been measured for butane [7-10] and cyclopentane [11,12].In previous work [13,14], we carried out a systematic study of the kinetics and selectivity of the reactions of OH radicals with a wide range of normal, iso, and cycloalkanes in the oxidative system containing H 2 O 2 , Fe 2+ , Fe 3+ , and water similar to Fenton's reagent [3]. We found unusually low substrate selectivity and hydrogen kinetic isotope effect (KIE) for reaction (1) in aqueous solution in comparison with the gas-phase reactions. These findings were qualitatively explained by diffusion complications for the reactions in solution related to the cage effect [13].A new approach toward the study of the mechanisms for activation of C-H bonds of saturated hydrocarbons by oxidizing agents, metal complexes, and electrophiles developed in previous work [13,15] involves study of the temperature dependence of the KIE for cyclohexane/cyclohexane-d 6 pairs and the ratios of the reaction rates for the cyclopentane/cyclohexane pair and analysis of the compensation ratios obtained. There have been no data on the temperature dependence of the rate constants of the reactions of alkanes with OH radicals in aqueous solution.