Crystalline silica was modified by annealing at temperatures ranging up to 1000 °C and by treating with a silane coupling agent (γ-glycidoxypropylotrietoxysilane) at concentrations up to 1 wt.%. The silica was dispersed in a matrix of diglycidyl ether of bisphenol-A epoxy resin. The dielectric properties of the composite samples were measured in the frequency range from 0.6–300 kHz. The results reveal that the dielectric loss index values of the samples are associated with water molecules adsorbed on the silica surface. An interpretation in terms of the cluster model of dielectric relaxation is provided. A relationship between the parameters pertaining to the silica modification and those describing the effects of the boiling water treatment on one hand and the dielectric loss index of the composites on the other has been determined.