Abstract: An oxidation process with water as the reaction medium might be effective in selective green oxidation of olefins and green oxidation treatment of organic compounds.The photooxidation of styrene by molecular oxygen was investigated in organic/water biphasic media at room temperature under UV light irradiation.The results indicated that the photoreaction, which occurred in aqueous medium, gave a high selectivity (90.8%) of the product, benzaldehyde, with a formative yield of 28.5%.The photoreaction which occurred in aqueous solution and organic solvents without molecular oxygen gave little amount of benzaldehyde with a yield of 0.2%-0.4%.The photooxidation process was also operative for the oxidation of several styrene derivatives (e.g., 3-methylstyrene, 4-methoxylstyrene and 1, 1-diphenylethylene), which can be selectively photooxidized into the corresponding aldehydes or ketones.The photoreaction which occurred in aqueous medium with hydrogen peroxide as oxidants gave benzaldehyde yield of 0.4%.However, the photoreaction which occurred in alcohol solvent with hydrogen peroxide or molecular oxygen as oxidants gave little amount of benzaldehyde with a yield of 0.6%-0.7%.The experiments indicated that both molecular oxygen and water are essential to photooxygenation reaction of styrene.