A study has been conducted on the effect of specific adsorption of o-chloranil. OCA, at an activated glassy carbon electrode, AGCE, surface on its electrochemical behavior in an aqueous solution. The voltammetric responses of OCA at an AGCE show an anodic prepeak and a cathodic postpeak other than the diffusion main couple in weak acidic and neutral pHs. The anodic prepeak and the cathodic postpeak are ascribed to specific adsorption of OCA. The pK(a) value of the adsorbed redox couple of OCA at an AGCE is at least by 4 and 2 pK(a) units higher than those of the free redox couple in solution at bare and activated glassy carbon electrodes, respectively. The slop of (I-p)(ads)/(I-p)(diff) ratio versus nu(1/2) for the cathodic and anodic peaks indicates a stronger adsorption of OCA at AGCE in comparison with OCAH(2). The effects of OCA concentration on its adsorption behavior and applied potential on anodic adsorption of OCA at AGCE have been studied and the surface coverage of OCA (Gamma(OCA)) has been calculated. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.