Electrode processes of hexaamineruthenium(III) and tris(1,10-phenanthroline)cobalt(II) were studied on various porous gold electrodes in I M NaNO3 and H2SO4. The results were compared with those obtained on the polycrystalline gold electrodes. Impedance spectra were analyzed and charge transfer resistances determined. For the redox reactions in solutions in some cases two semicircles were observed on the complex plane plots. The experimental results were compared with the theory of electrochemical processes taking place on the porous electrodes developed earlier [A. Lasia, J. Electroanal. Chem., 428 (1997) 155, A. Lasia, J. Electroanal. Chem., 500 (2001) 30], which predicts formation of two semicircles on the complex plane plots for a simple one-electron redox system. Their nature and occurrence is discussed. (C) 2005 Elsevier B.V. All rights reserved.