A series of inorganic-organic nanocomposites containing sulfonated polyimides and silica particles were synthesized. Anhydride-terminated pre-polymers were prepared, followed by introduction of reactive silanes. Casting, curing, and acidification routines lead to nanocomposites with significantly different properties compared to parent sulfonated polyimides. The presence of silica was qualitatively confirmed using energy dispersive X-ray spectrometry and studied using solid-state Si-29 NMR. Thermogravimetric analysis provided a more quantitative assessment of the inorganic fraction. Electron microscopy, water vapor sorption, and impedance studies were conducted to understand how silica and ion content influences morphology and proton conductivity. Ion clusters were visible following Ag+ staining, and silica nanoparticles were imaged in unstained samples. Silica nanoparticles significantly reduce the solubility of prepared membranes, and promote membrane hydration. For nanocomposites prepared from high molecular weight pre-polymers, silica incorporation promotes conductivity at low relative humidity. However, for nanocomposites made from low molecular weight or high ionic content prepolymers, silica dilutes the ion concentration and lowers proton conductivity. (C) 2009 Elsevier Ltd. All rights reserved.