Gas diffusion electrodes (GDEs) containing Pt/C catalyst and sulfonated poly (arylene ether) (SPAE) ionomer as a proton conducting binder (SPAE-GDE) were prepared. The cathode performances were evaluated in a PEFC at 80 degrees C and relative humidities (RH) from 60 to 100%. The value of the electrochemical active surface area (ECA) of 65 m(2) g(-1) for the SPAE-GDE. which was measured by cyclic voltammetry at 100%-RH and 40 degrees C, was nearly the same as that for a Nafion-GDE. In contrast, the RH-dependencies of the performance indices (Such as mass activity at 0.9 V, Tafel slope, or current density at 0.7 V) at the SPAE-GDE were very large and distinct from those for the conventional Nafion-GDE. With decreasing RH, the mass activity at the Nafion-GDE decreased monotonically, whereas that at the SPAE-GDE reached a maximum at 88%-RH and decreased steeply below 78%-RH. predominantly due to a strong adsorption of functional groups, probably sulfonate groups, in the SPAE on the Pt catalyst surface. However, at 88-100%-RH, the concentration polarization became significant for current densities j > 0.1 A cm(-2) due to excessive swelling of the SPAE ionomer. Strategies to improve the performance of SPAE-GDEs have been elucidated. (C) 2009 Elsevier Ltd. All rights reserved.