We report on the preparation and characterization of perfluorosulfonic acid ionomer (PFSA) - silica composite membranes using hyperbranched polyethoxysiloxane (PEOS) as silica precursor. The membranes were formed via an in situ sol gel process of PEOS in a PFSA solution and subsequent solution casting. Transmission electron microscopy and small-angle X-ray scattering data showed that at low silica content ultra-small silica particles with a size smaller than the diameter of the ionic channels were homogeneously dispersed in the PFSA matrix. Such morphology leads to enhanced proton mobility as probed by impedance spectroscopy and H-1 solid-state NMR, and eventually resulted in the improvement of fuel cell performance. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.