The amavadine complex and its model [VL(2)](2-) (L = (ON)-O--[CH(CH3)COO-](2) (HIDPA(3-)) or (ON)-O--(CH2COO-)(2) (HIDA(3-)), respectively) undergo, in aqueous medium and at a Pt electrode, a fully electrochemical and chemical reversible V-iv/v oxidation and act as electron-transfer mediators in the electrocatalytic oxidation of some thiols (HSR) such as HS(CH2)(n)COOH (n = 1 or 2, i.e., mercaptoacetic or mercaptopropionic acid, respectively) and HSCH2CH(NH2)COOH (cysteine) to the corresponding disulfides (RS-SR) which were isolated upon bulk preparative electrolyses. As shown by digital simulation of cyclic voltammetry, this redox catalysis process occurs through an unprecedented mechanism involving Michaelis-Menten type kinetics with formation (k(1) = 1.2 x 10(3) M(-1) s(-1)) of an intermediate species (with half-life time of ca. 0.3 s) derived from the interaction of the oxidized vanadium complex (the active form of the mediator) with the substrate. A possible biological role for amavadine is suggested by these results.