The electrochemical oxidation of 4,4'-biphenol was studied at a glassy carbon electrode in the presence of glutathione and N-acetylcysteine, using cyclic voltammetry as a diagnostic technique. The results indicate a complicated catalytic system. The catalytic current depends on the concentration of glutathione (or N-acetylcysteine) and solution's pH. In this work a unique kinetic zone diagram for catalytic behavior of 4,4'-biphenol in the presence of glutathione is constructed using the variables "excess factor" (gamma = C-GSH(0)/C-B(0)) and "kinetic parameter" (lambda = pH/v). This paper also describes the elucidation of second order homogeneous catalytic reaction (EC) of 4,4'-biphenol with glutathione and N-acetylcysteine. In addition, the observed homogeneous rate constant of catalytic reaction of electrochemically generated 4,4'-diphenoquinone with glutathione (or N-acetylcysteine) was investigated using digital simulation method. (C) 2011 Elsevier Ltd. All rights reserved.