Adsorption and electrooxidation of CO on polycrystalline Pt and Pt-Ru alloys (Pt0.5Ru0.5, Pt0.7Ru0.3) has been studied by means of in situ infrared (FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS). Within the resolution limits of this study (Delta R/R x 10(-3)) only one adsorbed CO(a) species at v approximate to 2070-2075 cm(-1) (Pt) and 2050-2060 cm(-1) (Pt-Ru) was detected. No significant difference in C-O frequency between Pt0.5Ru0.5 and Pt0.7Ru0.3 was found. The oxidation of CO adsorbed in the hydrogen region on Pt-Ru shows distinct differences when compared with a polycrystalline Pt electrode. Whereas on Pt the oxidation of CO proceeds in two stages, on Pt-Ru cyclic voltammetry shows only one oxidation peak at a less positive potential compared to Pt. The lowest peak potential for CO adsorbate oxidation was observed on Pt0.5Ru0.5 at about 380mV vs. rhe. The results for CO oxidation obtained on Pt and Pt0.5Ru0.5 deposited onto a porous Au substrate by on line mass spectrometry were in accordance with the ir spectroscopic data. It was shown that Ru atoms at the surface of a Pt-Ru bimetallic electrode increase the CO tolerance of the catalyst. The promoter effect of Ru is explained with the bifunctional mechanism for CO oxidation where oxygen containing species are bounded preferentially to ruthenium surface atoms.