The oxidation of Trolox C (a vitamin E analog) by the hydroperoxidase activity of lipoxygenase was studied. Trolox C was oxidized to its corresponding phenoxyl radical in the presence of hydrogen peroxide, evolving through a ketodiene intermediate to the Trolox C quinone. The H2O2/Trolox C quinone molar ratio was 1.0. The overall reaction followed an enzymatic-chemical second-order system and involved a substrate regeneration mechanism. From the equations derived from this mechanism, the dismutation constant of the Trolox C radical was evaluated by non-linear regression as 4 x 10(5) M(-1) x s(-1). The accumulation curve of Trolox C quinone was found to be linear, with no lag period, and dependent on enzyme concentration. No phenoxyl radical was detected when the reaction was carried out in the presence of ascorbate. This synergistic reaction between the Trolox C radical and ascorbate was quantitative and depended on the respective concentrations of enzyme, Trolox C and hydrogen peroxide. The results presented in this paper suggest that the diferences observed in the kinetic behaviour of monophenols (one-electron donors) and diphenols (two-electron donors) stem from the fact that the latter evolve directly into ferric form without taking the slow pathway once the steady state is reached, whereas the monophenols are always forced take the slow way, even in the steady state. This peroxidative oxidation of a vitamin E analog by the hydroperoxidase activity of lipoxygenase together with the oxidation produced by dioxygenase activity suggests that lipoxygenase might be a key enzyme in destroying the lipophilic antioxidant barrier against the reactive oxygen species in membranes.