Abstract— Oxygen effects on the photocyclization of N‐methyldiphenylamine to N‐methylcarbazole were investigated in n‐hexane, water, and aqueous surfactant solutions by steady state irradiations and flash photolysis measurements. The reaction sequence in micelles was found to involve the same intermediate steps as in homogeneous solutions. In aerated micellar solutions, the quantum yield of N‐methylcarbazole is significantly higher than in n‐hexane, while the rate constants of the unimolecular reaction steps show no solvent dependence. The bimolecular dehydrogenation of the intermediate 4a, 4b‐dihydro‐N‐methylcarbazole by oxygen is enhanced in aqueous and micellar solutions, whereas the quenching rate of triplet intermediates by oxygen was not affected. The lesuhs are interpreted using a dispersed phase model of micellar solutions. Special ‘micellar effects’ need not be invoked since the dependence of the quantum yield on the solvent is shown to be due to the difference in the overall oxygen concentration.