The electrochemical reduction of adrenochrome on a glassy carbon electrode was investigated by ac impedance in Various pH McIlvaine buffer solution of constant ionic strength. The plots of impedance on the complex plane were semicircles rotated clockwise around the origin in higher frequencies. In lower frequencies, the impedance spectra showed the transition of Warburg-to-capacitive behavior. In the corresponding admittance diagrams, distorted semicircles appear. The electrode reaction involves both adsorption of electroactive species and the effect of concentration polarization. Based upon the mathematical model of the equivalent circuit, the optimal estimated values of some model parameters were obtained under different pH and polarization potentials by using a Gauss-Newton algorithm and nonlinear curve fitting. The mechanism of the electrode process was suggested.