CO tolerance of carbon-supported Pt80Ru20, Pt80Os20 and Pt85Co15 electrocatalysts has been studied in 0.1 M perchloric acid solution applying cyclic voltammetry and in situ Fourier transform infrared spectroscopy (FTIRS), and the results were compared with those for a commercial Pt ETEK catalyst. CO stripping is used for the normalization of the area of the electrodes. It is shown that FTIR spectra can be acquired during CO oxidation at these technical materials in an electrochemical cell, obtaining valuable information on the reactivity, which allows predicting their behaviour in a polymer electrolyte membrane fuel cell (PEMFC). Linear adsorbed CO is the main adsorbate, but also small amounts of bridge bonded CO are formed. Moreover, COH species seem to be also present for Pt80Os20 and Pt85Co15. On the other hand, the onset potential for CO oxidation to CO2 is established and compared for the four catalysts. All bimetallic alloys are able to oxidise adsorbed CO at lower potentials than platinum and a shift of about 0.20 V is determined. (C) 2006 Elsevier B.V. All rights reserved.