Nuclear magnetic resonance spin-spin relaxation (T-2) measurements have been used to investigate the segmental mobility of high molecular weight poly(dimethylsiloxane) (PDMS) polymers when blended with trimethylsilyl-treated polysilicate nanoparticles. The relaxation behavior was complex and could not be described by a single-exponential function. The results showed that a high molecular weight polysilicate caused a dramatic reduction in the overall PDMS chain mobility above a critical concentration. However, a surprising increase in mobility was observed below this critical concentration. The origin of this phenomenon was hypothesized to be a combination of free volume, polymer adsorption, and entanglement effects at low particle concentrations. The apparent increase in mobility was also manifested by a decrease in the shear viscosity and plateau modulus relative to data for the base polymer. The critical concentration was found to be dependent upon temperature and molecular weight.