The effects of the amount of MnO2 (0.5-2 mol%) on the microstructure and the electrical properties have been studied in a binary ZnO-0.5mol% V2O5 system. The microstructure of the samples consists mainly of ZnO grains with gamma-Zn-3(VO4)(2) as the minority secondary phase. The cumulative addition of MnO2 improves the varistor performance by increasing the nonlinear coefficient alpha. The alpha-value is found to increase with the amount of MnO2 and a hi-hest value of 31.8 is obtained for the sample doped with 2 mol% MnO2. (C) 2002 Elsevier Science B.V. All rights reserved.