A new setup of an attenuated total reflection-Fourier transform infrared (ATR-FTIR) method involving a thin electrolyte layer was developed for the in situ investigation of electrode processes. The applicability of the approach was tested in 0.5 M H 2 SO 4 by studying the oxidation of adsorbed CO using polycrystalline Pt and Pt particles supported on high surface area carbon black. The sensitivity of this new ATR-FTIR method appears to be better than found for previously reported results obtained using external reflection-absorption FTIR. High sensitivity was obtained by making a very good contact between the electrode surface and the ATR crystal, which was achieved by using a spring, a micrometer and X- and Y-position controllers. This new thin electrolyte layer ATR-FTIR technique was also successfully applied to the in situ study of surface species adsorbed on an IR non-reflecting porous electrode.