Interfacial covalent bonding is an effective approach to increase the electrical resistance of a polymer-particle composite to charge flow and dielectric breakdown. A bifunctional tether reagent bonded to an inorganic oxide particle surface assists with particle dispersion within a thermosetting epoxy polymer matrix but then also reacts covalently with the polymer matrix. Bonding the particle surface to the polymer matrix resulted in a composite that maintained the pure polymer glass transition temperature, compared to modified or unmodified particle dispersions that lacked covalent bonding to the polymer matrix, which depressed the polymer glass transition to lower temperatures. The added interfacial control, directly bonding the particle to the polymer matrix, appears to prevent conductive percolation across particle surfaces that results in a reduced Maxwell-Wagner relaxation of the polymer-particle composite and a reduced sensitivity to a dielectric breakdown event. The inclusion of 5 vol% particles of higher permittivity produces a composite of enhanced dielectric constant and, with surface modification to permit surface cross-linking into the polymer, a polymer-particle composite with a Weibull E 0 dielectric breakdown strength of 25% greater than that of the pure polymer resulted. The estimated energy density for the cross-linked interface composite was improved 260% compared to the polymer alone, 560% better than a polymer-particle composite synthesized using bare particles, and 80% better than a polymer-particle composite utilizing bare particles with a dispersant. © Koninklijke Brill NV, Leiden, 2010.