The reduction of [Mo2(Cp)2(μ-SMe) 3(μ-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 [Mo2(Cp)2(μ-SMe)3(μ-OH)]+ (2+). This electrochemical process is followed by protonation of the neutral μ-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 [Mo2(Cp) 2(μ-SMe)3(μ-η1,η1- 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(μ-SMe)3(H2O)(TsO)] (4) and [Mo2(cp)2(μ-SMe)3(H2O)L] + (L = H2O or THF) (6+), all of which contain a terminal aqua ligand, has also been investigated. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.