Ring substituted cobalt phthalocyanine complexes of the form CoPc(X)(4), (where X = NH2, NO2, C(CH3)(3), SO3H and COOH) are compared for their catalytic activities towards the oxidation of cysteine. The potential for the electrocatalytic oxidation of cysteine is closely related to the Co-III/Co-II couple of the CoPc(X)(4) complexes in acidic media and to the Co-II/Co-I couple in basic media. The catalytic currents and the oxidation potential for cysteine are dependent on the pH of the solution, the potential becoming less positive as the pH increases and the catalytic currents decreasing with increase in pH, for the same concentration of cysteine. The magnitudes of the catalytic currents (after ten scans) for cysteine oxidation at pH 8.3 and on electrodes modified with the CoPc(X)(4) complexes increased with the nature of the ring substituent as follows: NO2 < NH2 < SO3H < C(CH3)(3) < COOH.