The effects of sintering time and rate of cooling on the microstructure and current/voltage characteristics of a ZnO varistor material have been investigated. The results provide information about the development of microstructure during sintering and have been interpreted in terms of the role which microstructure plays in the determination of electrical properties. A continuous network of intergranular Bi2O3, which lies mainly along the triple junctions between ZnO grains, can give a substantial contribution to the leakage current. The conductivity of this skeleton depends upon its internal microstructure.