The behavior of sulfonated poly(ether ether ketone) (sPEEK) membranes in ethanol-water systems was studied for possible application in direct ethanol fuel cells (DEFCs). Polymer membranes with different degrees of sulfonation were tested by means of uptake, swelling, and ethanol transport with dynamic measurements (liquid-liquid and liquid-gas systems). Ethanol permeability was determined in an liquid-liquid diffusion cell. For membranes with an ion-exchange capacity (IEC) between 1.15 and 1.75 mmol/g, the ethanol permeability varied between 5 x 10(-8) and 1 x 10(-6) cm(2)/s, being dependent on the measuring temperature. Ethanol and water transport in liquid-gas systems was tested with pervaporation as a function of IEC and temperature. Higher IEC accounted for higher fluxes and lower water/ethanol selectivity. The temperature had a large effect on the fluxes, but the selectivity remained constant. Further-more, the membranes were characterized with proton conductivity measurements. The proton diffusion coefficient was calculated, and a transition in the proton transfer mechanism was found at a water number of 12. Membranes with high IEC (>1.6 mmol/g) exhibited larger proton diffusion coefficients in ethanol-water systems than in water systems. The membrane with the lowest IEC exhibited the best proton transport to ethanol permeability selectivity. The use of sPEEK membranes in DEFC systems depends on possible modifications to stabilize the membranes in the higher conductive region rather than on modifications to increase the proton conductivity in the stable region. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2998-3009, 2009