Laser flash photolysis studies of the hydrogen atom abstraction reaction of triplet benzophenone from 2-propanol and 1, 4-cyclohexadiene in supercritical C02 reveal unusual pressure effects on absolute rate constants. Monitoring reactivity close to the critical temperature (T0) revealed that bimolecular rate constants increasesharply with a decrease in pressure, approaching the critical point. Kinetic investigations along an isotherm further removed from Tc and predictive calculations on the pressure effect expected in supercritical C02 indicatethat enhanced reactivity is due to local substrate clustering.