The electrochemical oxidation of hydroquinone has been investigated in acetonitrile solutions under acidic and basic conditions. In aprotic medium, the oxidation mechanism of pure hydroquinone follows an ECE sequence under a mixed kinetic control by electron and proton transfer. When a strong acid is present in the medium, the stabilization of the radical cation generated in the first oxidation step allows a mechanistic transition from ECE to DISP2. The oxidation mechanism is also sensitive to the presence of weak and strong hydrogen bonding acceptors, such as dimethylsulfoxide and the acetate ion. Under these conditions, the oxidation is less anodic, which can be explained by formation of association complexes in which proton transfer is possible after the electron transfer. It is shown that the oxidation wave observed in typical experiments of quinones in the presence of proton donors is attributed to hydroquinone associated with the conjugated base of the proton donor. Cyclic voltammetry, H-1 NMR spectrometry and X-ray diffraction were used to obtain the information here discussed. (C) 2007 Elsevier B.V. All rights reserved.