Model bimetallic catalyst surfaces were generated by depositing small submonolayer amounts of Ru, Sn and Mo on Pr(111) and Pt(332) single crystal electrodes. In particular, in the case of Pt(332), a step decoration was achieved and thus a catalyst surface with a known atomic arrangement of the constituents. Ternary catalysts were modelled by sequential deposition of Sn and Ru. To study the influence on the rate of oxidation of adsorbed CO in cyclic voltammetry, CO2 formation rates were monitored by differential electrochemical mass spectrometry (DEMS), which allows a separation from pseudocapacitive effects, e.g. oxygen adsorption. Contrary to PtRu alloy electrodes, adsorbed CO on the Ru modified single crystal electrodes is oxidized in two oxidation peaks. In accordance with Monte Carlo simulations [I], this is due to slow diffusion of adsorbed CO to Ru sites. The CO adsorption state corresponding to the 2nd peak is also not oxidized during an extended potential stop at the onset of the Ist peak. Ternary model catalysts were used to test whether a synergetic effect of Ru and Sn. which influence the CO oxidation guile differently, is possible. The ternary model catalyst behaved like a superposition of the corresponding binary catalysts, probably because separate 2D ton Pt(lll) or 1D ton Pt(332)) islands of Ru or Sn were formed instead of an atomically mixed overlayer. Mo shifts the onset potential for oxidation of adsorbed CO to even lower potentials (0.15 to 0.2 V) than sn. However, at such a low potential only about 10% of the adsorbate is oxidized, the main oxidation peak is hardly influenced. (C) 2000 Elsevier Science Ltd. All rights reserved.