Membrane electrode assemblies (MEAs) with a sulfonated polysulfone (sPSU) as the proton-conducting phase were fuel cell evaluated at varying temperatures in over-humidified conditions. The sPSU was prepared by a direct polycondensation involving a commercially available sulfonated naphthalene diol monomer. The gas diffusion electrodes (GDEs) and MEAs were successfully fabricated and a thorough morphological study was subsequently carried out on GDEs with varying sPSU contents and ink solvents. The scanning electron microscopy and porosimetry studies revealed highly porous GDE morphologies at sPSU contents below 20 wt %. Double-layer capacitance measurements showed an almost fully sPSU-wetted electronic phase when the sPSU content was 10 wt %. The MEAs were prepared by applying the GDEs directly onto sPSU membranes. MEAs with a total Pt loading of 0.2 mg/cm(2) were successfully fuel cell operated at 120 degrees C. The MEAs showed mass-transport limitations in the range of 600-800 mA/cm(2), most probably caused by abundant water due to the overhumidified measuring conditions. The low resistance of the MEAs indicated a well-integrated structure between the GDEs and the membrane. (c) 2006 The Electrochemical Society.