Nanocomposite proton exchange membranes were prepared from sulfonated poly (phthalazinone ether ketone) (sPPEK) and various amounts of sulfonated silica nanoparticles (silica-SO3H). The use of silica-SO3H compensates for the decrease in ion exchange capacity of membranes observed when non-sulfonated nano-fillers are utilized. The strong -SO3H/-SO3H interaction between sPPEK chains and silica-SO3H particles leads to ionic cross-linking in the membrane structure, which increases both the thermal stability and methanol resistance of the membranes. The membrane with 7.5 phr of silica-SO3H (phr=g of silica-SO3H/100 g of sPPEK in membranes) exhibits low methanol crossover, high bound-water content, and a proton conductivity of 3.6 fold increase to that of the pristine sPPEK membrane. (C) 2007 Elsevier B.V. All rights reserved.