The electrochemical reduction of 3,5-di-tert-butyl-1,2-benzoquinone (Q) has been investigated in acetonitrile with glassy carbon electrodes in the absence and presence of the hydrogen-bond and proton donating additives, water and 2,2,2-trifluoroethanol (TFE). Under nominally anhydrous conditions, the first step of the reduction is the reversible formation of the persistent radical anion, Q(-.). However, in long-term experiments such as controlled potential electrolysis, the radical anion disappears slowly with the rate being enhanced by the addition of water. This reaction was shown to be the water-promoted disproportionation of the radical anion giving neutral quinone (Q), the protonated dianion (HQ(-)), and hydroxide. This reaction is too slow to affect the voltammetric experiments. Variation of the standard potential for the first step with the addition of water was explained by the formation of hydrogen-bonded complexes between Q(-.) and water. The 1:1 complex, (Q(-.))(H2O), is proposed to be the reactant in the second step of the reduction and it is suggested that the reaction is a concerted electron and proton-transfer reaction in which insertion of the electron into the complex is concerted with transfer of a proton from water to the developing quinone dianion. The principal support of this suggestion is the observation of very small electron-transfer coefficients, cc, for the second process in the presence of both water (alpha = 0.20) and TFE (alpha = 0.14-0.18). The voltammograms were adequately accounted for by digital simulation using this mechanism. Finally, the extremely small size of the second reduction peak at low water concentrations has been explained by the rapid reaction of hydroxide, formed in the second step, with quinone that continues to arrive at the electrode. This reaction scheme adequately accounts for the steady-state voltammograms at a microelectrode. The product of the reaction of quinone and hydroxide is suggested to be a gem-diolate.