Novel hybrid composite membranes were prepared by blending poly(ethylene glycol) functionalized polyhedral oligomeric silsesquioxane [PPOSS] as nanofiller in varying concentration ranging from 1 to 5% (w/w) into sulfonated poly(ether ether ketone) [SPEEK] with degree of sulfonation similar to 55% for proton exchange membrane fuel cells [PEMFCs]. The effect of incorporation of PPOSS into SPEEK matrix was investigated in terms of thermomechanical and morphological properties, water uptake and proton conductivity of SPEEK. All the composite membranes were thermally and mechanically stable up to 250 c. Transmission electron microscopy (TEM) revealed that the smallest particle size (similar to 100 nm) of PPOSS was found for SPEEK membranes containing 2% (w/w) PPOSS where as agglomeration (similar to 300 nm) was observed at higher loadings of PPOSS. The proton conductivity was found to be dependent on the morphology and was independent of the amount of water present in the membranes. At 100 degrees C and 100% RH, the highest proton conductivity (47 mS/cm compared 34 mS/cm for neat SPEEK i.e. an increase of similar to 51%) was recorded at 2% (w/w) PPOSS contents followed by a decrease on further addition of PPOSS. The water uptake of composite membranes increased with concentration of PPOSS while maintaining their hydrolytic stability at 100 degrees C for more than 24 h. (C) 2013 Elsevier Ltd. All rights reserved.