In this work, systematic substitution of ZnO with SnO2 in ZnO-based varistor composition was investigated through analysis of densification, phase formation, microstructure and non-linear properties. The general formula was Zn0.96-xSnxBi0.02Co0.02Oβ with x = 0, 0.01, 0.02 and 0.04. The high rate of densification starting at 800°C is believed to be related to Bi2O3 via liquid-phase sintering. The complex nature of this multi-phasic varistor system was demonstrated through occurrence of different phases at different sintering temperatures. The low-temperature pyrochlore (Bi2Sn2O7) started to dissociate at approximately 1100°C with the spinel (Zn2SnO4) forming afterwards. The grain size variation with composition might be induced by the changes in the availability of Bi2O3 which has been reported to help facilitate abnormal grain growth. All samples sintered at 1200°C, with data extrapolation, possessed breakdown voltages of higher than 1400 V/cm.