The oxidation of 6-hydroxy-2,2,5,7,8-pentamethylchroman, Trolox C, and α-tocopherol by horseradish peroxidase was examined by stopped- flow and ESR experiments. The catalytic intermediate of horseradish peroxidase during the oxidation of vitamin E analogues and vitamin E was invariably Compound II, and rate constants for the rate-determining step decreased in the order 6-hydroxy-2,2,5,7,8-pentamethylchroman > Trolox C > α-tocopherol. The formation of phenoxyl radicals from substrates was verified with ESR and was followed optically. Resulting 6-hydroxy-2,2,5,7,8-pentamethylchroman and Trolox C radicals decayed through a dismutation reaction, followed by formation of the quinoid form via a transient intermediate. The sequence of events after formation of 6-hydroxy-2,2,5,7,8-pentamethylchroman and Trolox C radicals was similar to that observed by pulse radiolysis (Thomas M. J. and Bielski, B. H. J. (1989). J. Am. Chem. Soc. 111, 3315-3319). Final oxidation products of 6-hydroxy-2,2,5,7,8-pentamethylchroman and Trolox C were identified as the quinoid forms and were obtained quantitatively whether or not the analogue had a carboxyl or methyl group at the 2-position of chroman ring. In contrast, enzymatic oxidation of a-tocopherol gave α-tocopherol quinone in very low yield. Conversion of 6-hydroxy-2,2,5,7,8-pentamethylchroman, Trolox C, and α-tocopherol to the corresponding quinones was also catalyzed by metmyoglobin in a reaction completely inhibited by ascorbate. © 1992.