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The photooxidation of styrene by molecular oxygen was investigated in organic–water biphasic media at room temperature under UV light irradiation. The photoreaction gave a high selectivity (>90%) of the product, benzaldehyde, with a formative yield of 29%. This new method was also operative for the oxidation of several styrene derivatives, such as 3-methylstyrene, 4-methoxylstyrene, β-methylstyrene, and 1,1-diphenylethylene, into the corresponding aldehydes or ketones. It was demonstrated that both molecular oxygen and water were essential to these reactions. The mechanistic study revealed that the primary reaction involved mainly electron transfer from the excited styrene to triplet ground state molecular oxygen, forming the superoxide radical anion and styrene radical cation, followed by cleavage of the CC bond viaan adduct intermediate in the presence of water.