Organic.inorganic hybrid crosslinked membranes based on poly(ethylene glycol) (PEG) and sulfonated polyhedral oligomeric silsesquioxane (POSS-SO3H) with urethane crosslinks were prepared as candidate materials for a proton exchange membrane for direct methanol fuel cells (DMFC). Infrared (FT-IR) spectroscopy and ion exchange capacity measurements for the hybrid revealed the POSS is incorporated as a part of the crosslinked network. We found that proton conductivity increased and methanol permeability decreased with increasing POSS content in the hybrid membrane. In particular, our hybrid membranes demonstrated proton conductivities comparable to that of Nafion 117 while exhibiting lower methanol permeability as compared to Nafion 117. We postulate that the polar sulfonic acid groups of the incorporated sPOSS cages assemble to provide ion conduction paths while the hydrophobic portions of the same sPOSS cages combine to form a barrier to methanol permeation of the hybrid membrane.