Sulfonated poly(ether ether ketone)s (SPEEKs) with various sulfonation degrees were prepared and characterized for the intention of fuel cell applications. Two distinct water vapor activity regions characterized by different water vapor uptake behaviors were observed. Proton conductivity of SPEEK membranes increases with increasing sulfonation degrees and temperatures. SPEEK membranes with sulfonation degrees of 5 1 and 60% show proton conductivity higher than 0.01 S cm(-1) at temperatures higher than 40 degrees C with 100% relative humidity (RH). Relative humidity has a stronger effect on the proton conductivity of SPEEK membranes than that of Nafion (R) membranes. Fuel cell performance with SPEEK membranes was studied at various temperatures and relative humidities. Good fuel cell performance was obtained with a SPEEK-51 membrane at 80 degrees C, 100% relative humidity and ambient pressure. Both temperature and relative humidity have important influence to the cell resistances and performances. Cyclic voltammetry (CV) as well as hydrogen crossover profiles show distinct features under different relative humidity conditions. (c) 2005 Elsevier B.V. All rights reserved.