The effects of the oxide additives MnO2, Sb2O3 and Co3O4 commonly incorporated in commercial Bi2O3-doped ZnO varistors on the microstructure and the current-voltage characteristics of 0.5 mol% Pr6O11-doped ZnO varistors have been studied. A 1 mol% addition of Co3O4 to the ZnO-Pr6O11 binary system does not produce any additional secondary phases in addition to Pr oxides. In contrast to this, the addition of 1 mol% MnO2 complicates the simple two-phase microstructure of the 0.5 mol% Pr6O11-doped material by forming the perovskite-type structure Pr(Mn-1 - xZn(x))O-3. The addition of Sb2O3 produces the pyrochlore Pr3Zn2Sb3O14 and spinel Zn7Sb2O12 phases. Application of the Fresnel fringe technique and the diffuse dark field technique showed that there was an amorphous Pr-rich layer of thickness <2 nm residing between adjacent ZnO grains in all the samples studied.