This study reports on the role of both electron donor and electron acceptor substituents on the periphery of phthalocyanine cycle on the electrocatalytic activity of cobalt plithalocyanines for the oxidation of 2-aminoethanethiol (2-AET). By using these groups it is possible to modulate the Co(II)/(I) formal potential of the catalysts. The reported results show that the electrocatalytic activity of the adsorbed phthalocyanines containing electron-withdrawing groups decreases as the Co(II)/(I) formal potential of the catalyst becomes more positive, while electron-donating groups on the periphery of the ligand increase the catalytic activity. © 2005 Elsevier B.V. All rights reserved.