Silica and sulfonic acid functionalized silica were synthesized by condensation of appropriate precursors through a sol-gel approach. SPEEK with three different ion exchange capacities (1.35, 1.75 and 2.1 mequiv. g(-1)) were prepared by sulfonation of PEEK. Composite membranes with 5% and 10% additive loadings were prepared by solvent casting. Characterization by FTIR spectroscopy confirmed the presence of sulfonic acid groups in the functionalized silica additives. The agglomerate size of the additives was estimated by scanning electron microscopy to be between 2 and 5 mu m. The room temperature liquid water uptake of the membranes was evaluated. Water uptake increased with SPEEK IEC. Composite membranes exhibited lower water uptakes when compared to pure SPEEK. Proton conductivities of up to 0.05 S cm(-1) at 80 degrees C and 75% relative humidity and 0.02 S cm(-1) at 80 degrees C and 50% relative humidity were recorded for SPEEK composite membranes prepared using sulfonic acid functionalized silica. Hydrogen crossover through the membrane was determined through linear sweep voltammetry on membrane electrode assemblies (MEAs). Hydrogen crossover current densities for all the MEAs were on the order of 1-2 mA cm(-2). MEAs tested showed reasonable performance at 80 C and 75% and 50% relative humidities. (c) 2007 Elsevier B.V. All rights reserved.