SUMMARY In recent years, nanometaloxide/epoxy resin composites have attracted interest because of their wide application in the electrical industry. Research on certain nanocomposites indicates that they may be advantageously used in the electrical power industry and as novel dielectric materials. Earlier work in the author's group showed improvement in dielectric properties of nanocomposites prepared with nanosized alumina fillers, e.g. resistance to surface degradation, modification of complex permittivity at low frequencies. In this work, we study the dielectric permittivity of epoxy-based nanocomposites with various metal oxide fillers including spherical alumina (Al2O3), titania (TiO2) and zinc oxide (ZnO) particles. Dielectric spectroscopy measurements and analysis are carried out in the frequency range of 10−4 to 103 Hz and, in the temperature range of 25°C to 90°C. To understand the effect of water, measurements are carried out in humid environments of 25% and 75% RH. It is shown that the dielectric behaviour changes significantly at high temperatures and humidities, especially for the nanocomposites. Copyright © 2012 John Wiley & Sons, Ltd.