A novel differential electrochemical mass spectrometry (DEMS) cell has been developed to study gas diffusion electrodes (GDEs) used in fuel cells under operating conditions. In this way, catalytic and diffusion properties of the electrodes can be evaluated at the same time. Moreover, DEMS, allows the detection of volatile and gaseous products and intermediates generated in the electrochemical reactions with good sensitivity. In this way, CO2 conversion efficiencies are evaluated during alcohol oxidation reaction. In the present communication, the electrochemical behaviour towards hydrogen evolution and CO and alcohol electrooxidation at different platinum-based catalysts has been studied using the new DEMS cell configuration. The relative yields of CO2 and by-side products during methanol oxidation have been evaluated to determine the CO2 conversion efficiency. In addition, the diffusional properties of diverse GDEs have been considered. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.