The effects of different commercial carbon powders (Vulcan XC72R, Black Pearls 3700 and Shawiningan Black), on the performance of double-layered gas diffusion electrodes are reported. The active mass was catalysed with cobalt tetramethoxyphenylporphyrin. Various techniques such as impregnation, permeation, porosimetry, SEM observations and impedance spectroscopy were used to characterise the structure of the electrodes. The electrochemical behaviour was determined by means of polarisation curves in alkaline electrolyte. The catalytic activity and the physical characteristics of the carbon material strongly affect the performance and the life of the electrodes. The results significantly contribute to the proper selection of carbon materials and to optimisation of the electrode manufacturing technology. Besides the estimation of some structural parameters enables the correct simulation of electrode behaviour. (C) 2000 Elsevier Science Ltd. All rights reserved.