Pulsed ESR has been used to measure rate constants for the addition of hydrogen atoms to benzene as a function of temperature in aqueous solution. At 298 K we obtain k,(aq) = (1.1 + 0.1).109 M -' s-I.Thc average Arrhenius activation energy, E, = 19.1(6) kJ. mol-I, is essentially the same in solution as in the gas phase, but the frequency factor, represented by log(A/M-' s-') = 12.34(9), is 54 times higher than in thc gas. This may be cxplained in terms of solvation of reactants and transition state, with most of the enhancemant attributed to H atom solvation. In a 90% DzO/lO% H 2 0 mix, H atoms react at the same rate, but deuterium atoms react more slowly than H atoms by a factor of 1.4. The much smaller solvent enhancement previously demonstrated for the light muonium isotope is tentatively attributed to incomplete hydration of the transition state in that case.