The use of sulfonated poly(ether ether ketone) as the proton-conducting medium within the electrocatalytic layer of fuel cell gas diffusion electrodes (GDEs) is described. An electrochemically active surface area determined by cyclic voltammetry and electrochemical impedance spectroscopy was studied as a function of ion exchange capacity. Maximum activity is related to the concentration of sulfonate groups within the catalyst layer. Despite their relatively high electrochemical surface area and catalyst utilization, fuel cell performance is rather poor compared to Nafion-based GDEs. Fuel cell performance is greatly enhanced when polytetrafluoroethylene (PTFE) is incorporated into the catalyst layer. The addition of PTFE results in GDEs with larger pore diameters that are less prone to flooding. The incorporation of an ionomer of similar chemical structure to the proton- conducting membrane employed is shown to reduce interfacial resistance. Electrode structures of this type may prove useful for investigating other novel proton- conducting materials. (c) 2005 The Electrochemical Society.