Organic-inorganic hybrid nanocomposite membranes were prepared using three different types of POSS i.e., PEG POSS (R) cage mixture (PPOSS), trisilanol phenyl POSS (R) (TSP POSS) and trisulfonic acid isobutyl POSS (R) (SPOSS) at a fixed loading of 2% (w/w) as filler and SPEEK with degree of sulfonation (DS) 55% as polymer matrix. The influence of POSS functionality on hybrid membrane's thermo-mechanical properties, morphology, water uptake and proton conductivity was investigated. Thermal and mechanical stability of hybrid membranes increased upon incorporation of POSS. The size and distribution of POSS particles into SPEEK matrix was studied using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) and it was found that TSP POSS and PPOSS based membranes showed smaller particle size and uniform distribution as compared to SPOSS based membranes which consequently affect the water uptake and proton conductivity of these hybrid membranes. The water uptake studies were carried out at three different temperatures i.e. 30, 80, 100 degrees C for 24 h and POSS based composite membranes showed higher water uptake and proton conductivity compared to neat SPEEK membranes. The highest proton conductivity (64.6 mS/cm) was observed for TSP POSS containing membrane which is more than double of neat SPEEK (31.3 mS/cm) membrane. The composite membrane containing TSP POSS can be considered as suitable membrane for PEMFCs applications. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.