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” ( γ = C GSH 0 / C B 0 ) and “kinetic parameter” ( λ = 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.