Polymer nanocomposite membranes based on sulfonated poly(ether ether ketone) (SPEEK) (degree of sulfonation 60%) containing varying amounts of open cage trisilanol phenyl polyhedral oligomeric silsesquioxane (FOSS) ranging from 0.5 to 5 phr (parts per hundred parts of the polymer resin) were prepared and characterized. The composite membranes were characterized by water uptake, proton conductivity, methanol permeability, X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The presence of nanofillers in the composite films was confirmed by XRD and SEM-EDX. The domain size of FOSS (for 0.5 phr sample) as determined by SEM was in the range of 50-100 nm. Thermal stability of SPEEK remained unaffected by the addition of varying amounts of POSS. Equilibrium water uptake of all the composite membranes (maximum 42%) was higher than that of pure SPEEK membrane (23%) at 80 degrees C. Proton conductivity of SPEEK as measured by two probe method increased upon incorporation of FOSS and maximum value was obtained when the POSS content was 2 phr (w/w), that is, 4.5 mS/cm, which was higher when compared with SPEEK (1.5 mS/cm) and Nafion (3.4 mS/cm) at 90 degrees C. Methanol permeability of composite membranes was lower than that of SPEEK/Nafion membrane. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 3013-3023,2010