Styrene oxide (yield 18-25%) and benzaldehyde (yield 40-45%) are the main products of te oxidation of styrene with molecular oxygen at 90 degrees C in toluene as the solvent. The yield of styrene oxide increases, that of benzaldehyde decreases with increasing conversion. Addition of benzaldehyde at the beginning of the oxidation increases the reaction rate and the yield of both styrene oxide and ester compounds. Gaschromatographic examination of the crude oxidation mixture showed the presence of 15 oxygen containing by-products which were quantitatively and qualitatively analysed. Pathways of by-product formation by cooxidation of styrene with benzaldehyde and toluene with participation of benzoyl, benzoyloxy, benzoylperoxy or benzyl radicals are proposed.