The dynamic relaxation characteristics of poly(ether imide) and poly(methyl methacrylate) nanocomposites based on native and surface-modified (i.e., hydrophobic) fumed silicas were investigated by dynamic mechanical analysis and dielectric spectroscopy. The nanocomposites displayed a dual glass transition behavior in the dynamic mechanical studies encompassing a bulk polymer glass transition (close to T-g for the unfilled polymer), and a second, higher-temperature transition reflecting relaxation of polymer chain segments constrained owing to their proximity to the particle surface. The position and intensity of the higher-temperature transition varied with particle loading and surface chemistry, and reflected the relative populations of segments constrained or immobilized at the particle-polymer interface. Dielectric measurements, which were used to probe the time temperature response across the local sub-glass relaxations, indicated no variation in relaxation characteristics with particle loading. (C) 2010 Elsevier Ltd. All rights reserved.