The reduction of [Mo-2(CP)(2)(mu-SMe)(3)(mu-O)](+) (1(+)) has been investigated by cyclic voltammetry and controlled-potential electrolysis in THF- and MeCN/NBu4PF6 without added acid or in the presence of various acids HX (HX: HTsO, CF3CO2H, HBF4). Reduction in the presence of acid follows an ECrevE mechanism in which the intermediate chemical step is an acid-base equilibrium between 1 and [Mo-2(Cp)(2)(mu-SMe)(3)(mu-OH)](+) (2(+)). This electrochemical process is followed by protonation of the neutral V-hydroxo complex 2 to afford different products which depend both on the solvent (THF or MeCN) and on the nature of the acid. Controlled-potential electrolysis of 1(+) in the presence of HX (2 equiv.) leads to the generation of binding sites and finally gives products identical to those obtained from protonation of 2 by HX. The complex [Mo-2(Cp)(2)(mu-SMe)(3)(mu-eta(1),eta(1)-OCOCF3)] (3) which can be obtained either by protonation of 2 by CF3CO2H or by reduction of 1(+) in the presence,of CF3CO2H, has been characterized crystallographically. In the presence of HBF4 protonation of 1(+) gives 2(2+). The reactivity of 2(+) and of the complexes [Mo-2(Cp)(2)(mu-SMe)(3)(H2O)(TsO)] (4) and [Mo-2(Cp)(2)(mu-SMe)(3)(H2O)L](+) (L = H2O or THF) (6(+)), all of which contain a terminal aqua ligand, has also been investigated. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).