The permeability of carbon dioxide and oxygen in Nafion((R)) 1100 EW membrane has been studied as function of the relative humidity or water content using a mass spectrometer equipped with a membrane inlet. The rate of permeation of CO2 was found to depend strongly on the water content of the membrane. It was similar to the oxygen permeation rate for the dry membrane but an order of magnitude higher for the fully humidified membrane. The data were used to correct the estimated methanol crossover rate and fuel efficiency in a commercial DMFC using Nafion((R)) 117 as electrolyte by subtracting the contribution from the CO2 permeation to the CO2 flux from the cathode. The contribution varied between 4.5% and 12% of the flux depending on the mode of operation. In an operating DMFC the electrolyte/anode interface will be saturated with CO2. For very efficient anodes (almost complete methanol conversion at the electrode/electrolyte interface) and the use of thin electrolyte membranes negligence of the CO2 permeation contribution may thus lead to misleading conclusions regarding the magnitude of methanol crossover. (c) 2005 Elsevier B.V. All rights reserved.