We report in this article the results of nanosilica (SiO(2))-filled epoxy composites with different loadings and their electrical, thermal, mechanical, and free-volume properties characterized with different techniques. The morphological features were studied by transmission electron microscopy, and differential scanning calorimetry was used to investigate the glass-transition temperature (T(g)) of the nanocomposites. The properties of the nanocomposites showed that the electrical resistivity (rho), ultimate tensile strength, and hardness of the composites increased with SiO(2) weight fraction up to 10 wt % and decreased there-after; this suggested that the beneficial properties occurred up to this weight fraction. The temperature and seawater aging had a negative influence on rho; that is, rho decreased with increases in the temperature and aging. The free-volume changes (microstructural) in the composite systems correlated with seawater aging but did not correlate so well with the mechanical properties. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 121: 2752-2760, 2011