High temperature (100-200 °C) operation of fuel cells alleviates CO poisoning of the electrocatalysts and thermal-management problems associated with current proton exchange membrane (PEM) fuel cell technology. Perfluorosulfonic acid-based membranes have high proton conductivity below 80 °C, but at higher temperatures conductivity and fuel cell performance rapidly dwindle. Incorporation of inorganic fillers into the PEM has been shown to increase its working temperature range and improve its mechanical properties. These include composite materials prepared by in-situ polymerization of tetraalkoxysilanes in Nafion®. However, these studies did not examine the effects of particle size and the surface chemistry of the silica on processing and composite properties. In this work, we prepared and characterized a family of nanocomposites of Nafion® with monodisperse silica spheres with varied particle size and surface chemistry. Silica particles with controlled sizes (20-200 nm diameter) were prepared from tetraethoxysilane by the Stöeber method. Surface modification with mercaptopropyltriethoxysilane afforded hydrophobic thiol-modified silica that, upon oxidation with hydrogen peroxide, were converted into hydrophilic, sulfonic acid modified particles. We were successful in developing procedures for homogeneously dispersing the particles in Nafion® to make 80-100μm thick nanocomposite membranes. Atomic force microscopy and scanning electron microscopy were used to determine the distribution of the silica particles in the membranes and how it is affected by their size and surface chemistry. The results of our morphological studies and the influence of the size and chemical modification of the well-defined silica particles on the properties of the composite membranes, such as water uptake and proton conductivity at high temperatures, will be presented.