We present the first comparative study of O-17 and O-18 isotope effects in a dioxygen-dependent enzymatic reaction. The binding of dioxygen to soybean lipoxygenase is expected to be near the diffusive limit, introducing the possibility of a magnetic isotope effect with O-17 reacting faster than O-16. However, the experimental data indicate a purely mass effect among the O-16, O-17, and O-18 isotopes. A reaction mechanism for soybean lipoxygenase is proposed in which dioxygen undergoes reversible reaction with a substrate derived radical, followed by a slow release of the bound lipid hydroperoxide.