The addition of a nonlinear filler in the form of particulate zinc oxide to insulating materials has been found to produce a marked nonlinear conductivity in the resulting composite. While retaining its excellent insulating properties at low electric fields, this composite material acquires a measure of conductivity at high fields. The paper investigates the influence of this phenomenon on a number of characteristics of insulating materials, specifically. bulk conductivity, surface charge dissipation, electrical treeing and tracking. Models have been developed for simulation of the effects of adding the nonlinear filler to polyester resin. A. good correlation exists between the theoretical models for surface charge dissipation and treeing resistance, modified to incorporate the nonlinear characteristic, and experimental data derived from polyester resin containing a range of concentrations of zinc oxide. For tracking, a contrary result was obtained which has been attributed to changes in the: mechanical characteristics of the filled material. An improvement in the dissipation of surface charge and treeing resistance has been demonstrated without loss of the material's insulating properties under normal electric fields.