O-2 reduction has been investigated using rotating disc electrode and voltammetric techniques for naphthalocyanine CoNPc(2,3) impregnations supported on two kinds of carbon blacks according to the same procedure as that described by A. Biloul, O. Contamin, G. Scarbeck, M. Savy, B. Palys, J. Riga and J. Verbist, J. Electroanal. Chem., 365 (1994) 239. Samples were also characterized by Fourier transform IR and X-ray photoelectron spectroscopy techniques which indicate a bilayer structure. An optimized thermal treatment yielded initial activities almost identical with those of the NPcCo(1,2) isomer mixture. However, after a 10 day stay in acidic solutions, the activities dropped in all cases and became comparable with those corresponding to non-thermally treated samples. Thus the isomer structural effect is relatively minor with CoNPc.