Three novel polyhedral oligosilsesquioxane (POSS) nanofillers functionalized with proton-conducting sulfonic acid groups, mixed sulfonic acid and alkyl groups, and phosphonic acid groups were synthesized, characterized by IR, H-1 and C-13 NMR, and MALDI-TOF MS, and formulated into sulfonated polyphenylsulfone (S-PPSU) carrier polymers. High quality films were cast from 1-methyl-2-pyrrolidinone (NMP), and through-plane and in-plane proton conductivity, mechanical properties, water uptake, dimensional stability, and leaching behavior were measured to assess their suitability for use as hydrogen fuel cell proton exchange membranes. Various nano-filler loadings and S-PPSU sulfonation levels were studied. The morphologies of the composite membranes were determined by TEM and SEM X-ray mapping. When compared with NAfion(R), the POSS-S-PPSU composite membranes exhibited comparable proton conductivity in combination with superior dimensional stability, heat resistance, and mechanical strength. When compared with control S-PSSU membranes, the composite POSS-S-PPSU membranes exhibited superior conductivity, comparabel dimensional stability, and slightly decreased mechanical strength. (c) 2008 Wiley Periodicals, Inc.