Nafion (R) /organically modified silicate (ORMOSIL) hybrids were generated via polymer in situ sol-gel copolymerizations of tetraethylorthosilicate (TEOS) with difunctional and trifunctional organoalkoxysilane monomers, and the dynamic mechanical relaxations and thermomechanical stability of the resultant composites were investigated. All ORMOSIL fillers restrict main-chain and side-chain mobility. Tile results suggest that sulfonic acid side chains become entrapped in silicate or ORMOSIL structures that evolve during the in situ sol-gel process. Some but not all of the ORMOSIL combinations display trends in the Lu or P relaxation temperature with respect to the TEOS comonomer ratio. All of the hybrids have greater high-temperature thermomechanical stability than the unfilled acid form. There are definite composition trends in the vertical displacement of tile storage-modulus-temperature curves. For some of the semiorganic comonomers, there is a monotonic increase in storage modulus with decreasing comonomer content in the high-temperature regime, which is understood in terms of a progressive immobilization of the long side chains by progressively more rigid ORMOSIL nanostructures. For other semiorganic comonomers, the high-temperature plateau shifts upward to a maximum and then shifts downward with decreasing comonomer fraction. In addition to the chemical nature of the imparted nanophases, these molecular motions are expected to influence the transport properties of these novel heterogeneous membranes.