CO adsorption and electro-oxidation on a Pt(111) electrode modified by Ru deposition was studied by IR spectroscopy. The IR spectra in the region of the CO stretching vibration exhibit distinct bands which can be assigned to CO adsorbed on the Pt substrate (COPt) and on Ru islands (CORu). This is in contrast with IR studies of Pt - Ru alloy electrodes where only a single vibrational band of adsorbed CO is observed. The integrated band intensities of the CORu and of COPt bands on the Ru-modified Pt(111) electrode decrease simultaneously at the onset potential of CO oxidation. The results are interpreted within the framework of a bifunctional mechanism of CO electo-oxidation, postulating active Pt sites in close proximity to the Ru islands and a high mobility of the COPt adsorbate layer. The differences in the IR spectra between the RuPt(111) electrode and the Pt - Ru alloys are explained by disparate surface structures (Ru islands versus homogeneous distribution of Pt and Ru atoms).