Sulfonated poly(ether ether ketone) (PEEK) and their composites are considered one of the most promising alternatives for Nafion, the industry benchmark for electrolytic membranes in proton exchange membrane (PEM) fuel cells. In the present study, PEEK was non-homogeneously sulfonated using concentrated H2SO4 at different temperatures (room temperature, 60 degrees C, and 80 degrees C) and time durations (5, 7, 48, and 72 h). Composite membranes of SPEEK with different weight ratios of AlPO4 synthesized by sol-gel were also prepared. Depending on the degree of sulfonation (DS), the Ion Exchange Capacity (IEC) of the membranes varied from 1.06 to 2.9 meq g(-1). XRD results show the increasing amorphous nature of the membranes with increase in IEC and DS value. The water uptake of the membranes also increased with DS. Simultaneous TGA-FTIR measurement of the composite membranes showed better thermal stability compared to pure SPEEK membranes. The water uptake and proton conductivity of the composite SPEEK membranes were found to be lower than that of pure SPEEK membranes, while the composite membranes exhibited a better swelling behavior and mechanical stability than the pure SPEEK samples. (C) 2012 Elsevier B.V. All rights reserved.