New classes of fluorinated polymer-polysilsesquioxane nanocomposites have been designed and synthesized. The synthesis method includes radical polymerization using the functional benzoyl peroxide initiator for the telechelic fluorinated polymers with perfluorosulfonic acids in the side chains and a subsequent in situ sol gel condensation of the prepared triethoxylsilane-terminated fluorinated polymers with oxide precursors. The telechelic polymer and nanocomposites have been carefully characterized by H-1 and F-19 NMR, FTIR, TGA, and TEM. The ion-exchange capacity (IEC), water uptake, the state of the absorbed water, and transport properties of the composite membranes have been extensively studied as a function of the content and structure of the fillers. Unlike the conventional Nafion/silica composites, the proton conductivity of the prepared membranes increases steadily with the addition of small amounts of the poly-silsesquioxane fillers. In particular, the sulfopropylated polysilsesquioxane-based nanocomposites display proton conductivities greater than Nafion. This is attributed to the presence of pendant sulfonic acids in the fillers, which increases IEC and offers continuous proton transport channels between the fillers and the polymer matrix. The methanol permeability of the prepared membranes has also been examined. Lower methanol permeability and higher electrochemical selectivity than those of Nafion have been demonstrated in the polysilsesquioxane-based nanocomposites. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4800-4810,2010