Using a novel combined spectro-electrochemical DEMS/ATR-FTIRS technique, the CO adsorption kinetics on a Pt film electrode were studied, performing transient CO adsorption experiments at different constant potentials (0.06-0.6V). CO adsorption rate and COad coverage were determined continuously from the CO consumption measured by on-line differential electrochemical mass spectrometry (DEMS). Simultaneously measured FTIR spectra, recorded in situ in an attenuated total reflection (ATR) configuration, allow a direct correlation of the IR band intensity and frequency with COad surface coverage at different constant potentials. The data show that (i) the CO adsorption kinetics are independent of the adsorption potential up to 0.5 V, (ii) a significant potential dependence of the ratio between COL and COM for the same coverage, (iii) in the regime of very high COad coverages there is no proportional relation between COad coverage and COL,M intensity, and (iv) a distinct tendency for COad island formation at E-ads < 0.2 V and > 0.4 V, most likely due to coadsorption of H-upd at the lower potentials and (bi-)sulfate at higher potentials. Finally, at 0.6 V, COad oxidation follows a Langmuir-Hinshelwood mechanism with the highest CO2 formation rate at a relative COad coverage of similar to 0.4. (c) 2007 Elsevier Ltd. All rights reserved.