Magnetic fields (0.3–0.4 T) were found to accelerate the oxidation of styrene and ethylbenzene with molecular oxygen by 10–70% with increasing the rate of initiation and decreasing the rate of chain termination. The effects are consistently explained in terms of triplet–singlet spin conversion in radical pairs, and they can be used to elucidate the oxidation mechanisms of biologically important molecules (polyunsaturated acids and lipids).