The electrochemical behaviour of fuel cell catalysts (mesoporous Pt (MPPt), MPPtRu, MPPt modified by adsorbed Ru (MPPt/Ru) and carbon supported PtRu alloy) was studied using the thin layer flow cell differential electrochemical mass spectrometry (TLFC-DEMS) technique. The catalysts present high catalytic activity towards the methanol oxidation reaction (MOR). being the PtRu/C electrode the least active for MOR, while MPPt/Ru presents higher current densities for this reaction than MPPtRu. The results suggest that the diffusion properties obtained in the porous structure of the MP electrodes and the surface atomic arrangement in the electrode are the main reasons for the higher catalytic activity achieved. Finally. TLFC-DEMS was proved to be a powerful technique which evaluates and correlates the CO2 efficiency with the catalytic activity and the porous structure of the catalysts. (C) 2010 Elsevier B.V. All rights reserved.