Styrene is oxidized to styrene oxide and benzaldehyde when incubated with horseradish peroxidase, H2O2, and 4-methylphenol. Styrene oxide is not formed in the absence of any of these reaction components or of molecular oxygen. The coupling products 2-(4-methylphenoxy)-1-phenylethane, 2-(4-methylphenoxy)-1-phenylethan-1-ol, and 2-(4-methylphenoxy)-2-phenylethan-1-ol are not formed, but the ortho-linked dimer of 4-methylphenol is a major product. The epoxide oxygen is labeled in the presence of 18O2 but not H218O2. Styrene oxide formation is not inhibited by mannitol or superoxide dismutase. The stereochemistry of trans-[1-2H]styrene is partially scrambled in the epoxide product. EPR signals attributable to the 2,4-dihydroxy-5-methylphenoxy radical, a product of the oxidation of 4-methylcatechol, are observed if Zn2+ is added to stabilize the radical. This radical is only detected in the presence of styrene. The results imply that styrene is epoxidized by the hydroperoxy radical generated by addition of molecular oxygen to the 4-methylphenoxy radical. The epoxidation mimics the chemistry proposed to occur in the protein-mediated cooxidation of styrene by hemoglobin and myoglobin.