Styrenated polyester nanocomposites are a class of polymers reinforced with low quantities of well-dispersed nanoparticles with the aim of offering advantages over conventional composites. Nanosilica fumes with different particle sizes were used as a filler to form polyester nanocomposites. The average particle size was detected with transmission electron microscopy. We used the electrical properties [permittivity (epsilon'), dielectric loss (epsilon ''), and electrical conductivity (sigma)] to investigate the variation of the properties of the polyester/nanosilica fume composites by varying the filler ratio and particle size as well. The epsilon', epsilon '', and sigma values were found to increase with increasing filler content up to a certain concentration, at which aggregation began to form and after which stability in these properties was attained. The second relaxation time (tau(2)) associated with the orientation of the aggregates caused by the movement of the main chain was found to be unchanged up to a certain concentration of filler, after which a pronouncing increase in tau(2) was noticed. This result found further justification through the scanning electron micrographs. In addition, the values of sigma, which were of the order of 10 (11) S/cm, make the materials highly recommended for use for insulation purposes. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 714-721, 2011