Crystalline phase formation and microstructure of ZnO varistors with a basic composition ZnO-Bi2O3-Sb2O3 were examined. Addition of chromium oxide to this basic varistor resulted in an alpha-spinel (alpha-Zn7Sb2O12) phase dissolving a significant amount of Cr, while the beta-spinel did not. beta-spinel transformed to pyrochlore during cooling, whereas alpha-spinel hardly transformed to pyrochlore irrespective of the cooling conditions. When Sb2O3 was completely replaced by Cr2O3, ZnCr2O4 was formed instead of spinel. alpha-spinel particles were 1-2 mu m in size and intra- as well as intergranular. ZnCr2O4 particles, smaller than 1 mu m in size, however, were present as aggregates in the bismuth-rich matrix phase at the grain boundaries.