In situ Fourier transform infrared-diffuse reflection spectroscopy is introduced as a method for studying the catalytic surfaces of membrane electrode assemblies in working fuel cells. An annular collection electrode and a CaF2 window are used to expose a sufficient area of the electrode to the IR beam. Experimental results for methanol oxidation, CO adsorption, and (CO2 + CO) adsorption show that this method can be used to monitor the catalyst surface in a direct methanol fuel cell under load. The IR peaks associated with CO adsorption on high surface area Pt-Ru are similar to previously observed peaks on smooth electrodes reported in the literature.