The electrochemical reduction of adrenochrome on a glassy carbon electrode was investigated by ac impedance in a constant ionic strength McIlvaine buffer solution. When the frequency was lower than 60 kHz, -Z " against Z＇ showed straight lines whose inclined angles with the real axis were within the range 71-76 degrees in the diagrams of the complex impedance plane obtained at different de potentials. In the corresponding admittance diagrams, distorted semicircles appeared. The electrode reaction involves adsorption of electroactive species and the effect of concentration polarization cannot be neglected. Based upon the admittance model of the equivalent circuit, the optimal estimated values of some model parameters were obtained by using a Gauss-Newton algorithm and non-linear curve fitting. The characteristics of the electrode process are also discussed.