Benzene photo-oxygenation reactions have been studied under CO2 pressurized conditions with the irradiation of KrF excimer laser light at 248 nm. The absorption measurements of the relevant mixtures indicate that the main photo-absorber is benzene. The identified principal product is phenol with smaller amounts of dihydroxybenzenes, whose quantum yield is 0.004 under a typical reaction condition in the neighbourhood of the critical point. The pulse energy dependence of the quantum yield indicates that the reaction is induced by one-photon absorption. A relatively flat quantum yield dependence on the total pressure has been found. The subsequent reaction progress is discussed on the basis of the quantum yield dependence on benzene and O-2 concentrations.