The halide-mediated peroxidase-catalyzed oxidation of ?-carotene was studied as a model system for lipid peroxidation. Of the halide ions tested, Br-, I-, and SCN- all caused some degree of ?-carotene oxidation at pH 7.0 and 4.4. No oxidation was observed with Cl-, however, which is consistent with Cl- not being a substrate for lactoperoxidase. The ?-carotene oxidation caused by the lactoperoxidase-I--H2O2 system was pH dependent, the rate of oxidation being 100-fold higher at pH 4.4 than at pH 7.0. When both I- and SCN- were present, the oxidation rate was reduced. At least partially, this interference between the two substrates was due to competition for the substrate-binding sites on the enzyme. © 1986 American Chemical Society.