The electrochemical properties of bismuth and germanium irreversibly adsorbed on platinum electrodes have been used to screen the surface domains of polycrystalline electrodes. As revealed by the use of stepped surfaces, bismuth adsorbed on (111) terrace sites shows a redox peak at 0.63 V, whose charge is proportional to the number of (111) terrace sites, without any interference from bismuth adsorbed on any other site. For the characterization of (10 0) terrace sites, adsorption of germanium can be used, since the adatoms adsorbed on the (10 0) terrace sites show a redox peak at 0.55 V. Although no other peak is found in this potential region for the germanium adsorbed on other sites, other contributions may give rise to an increase in the current in this region. After the appropriate corrections, the charge for bismuth and germanium redox process in the selected region is proportional to the number of terrace sites with (1 11) and (10 0) symmetry, respectively. These relationships can be used to characterize the bidimensional domains of any platinum electrode. Four different platinum nanoparticle electrodes were characterized and the results were used to understand their behaviour towards ammonia oxidation. (C) 2005 Elsevier Ltd. All rights reserved.