The effect of the addition of various amounts of Pr6O11 on the grain growth and densification process, microstructure, formation and distribution of secondary phases, and electrical properties of SnO2-based varistors containing cobalt and niobium oxides has been investigated. It has been observed that in the presence of as low as 0.1 mol% of Pr6O11, the formation of Pr2Sn2O7 can be detected using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The kinetics of grain growth in samples without and with praseodymium oxide has shown that increasing Pr6O11 from 0 to 0.5 mol%, changes the grain growth exponent from 2 to 6 and the activation energy from 350 kj/mol to around 700 kj/mol. This may be due to the cooperative effect of the secondary phases. The largest nonlinear coefficient, alpha, and the lowest leakage current, I-L, has been observed for the sample containing 0.05 mol% Pr6O11 whereas the highest breakdown voltage has been recorded for the sample with 0.5 mol% Pr6O11. (C) 2011 Elsevier B.V. All rights reserved.