The determination of the surface area of Pt and Ru electrocatalyst surfaces by oxidation of adsorbed CO and by oxidation of a Cu upd layer are compared. The amount of adsorbed CO was determined mass-spectrometrically from the ionic current for CO2 formation during an oxidative potential sweep. On Ru, the Faradaic charge is too large (by approx. 55%) due to Faradaic effects (oxygen adsorption). For massive Ru electrodes a Cu upd charge of 520 mu C cm(-2) is found after normalization to the area determined by CO oxidation. Using this value, both methods yield identical surface areas for nanoparticulate Ru catalysts. On Ru surfaces (both massive and nanoparticulate) completely covered by Se the amount of Cu upd charge decreases to one fourth of the value observed for pure Ru. Since CO is only adsorbed on free Ru sites and not on Se covered sites, the oxidation charge for the latter can be used to determine the number of free Ru sites, whereas the decrease of the Cu upd charge on Se modified surfaces can be used to calculate the area which is modified by Se. This method, previously tested on the model electrodes, was extended to Ru nanoparticle and Ru/Se electrodes. Using this surface determination it is possible to draw conclusions about the active surface area and the Se composition of the outer shell of Ru/Se nanoparticles. For the first time we also show, using RRDE measurements, that the oxygen reduction reaction is enhanced by simple Se adsorption also on massive Ru. It could be shown that the activity for the Ru/Se electrode increases with the Se amount on the surface.