The breakdown electric field (E_B) versus thickness (d) of the metal oxide varistor (MOV) relation is found to be closely related to the degree of microstructural heterogeneity in ZnO–Bi₂O₃ based varistors. For traditional electroceramic processed samples with various E_B there exists a critical inflection point behaviour in the E_B versus d plot. However, no critical inflection point was observed for amine processed MOV samples. The scanning electron microscopy shows amine processing largely improves uniformity in the grain size distribution. The dielectric spectra as a function of temperature yielded a thermal activation energy of 0.36 eV for the amine samples. Similar investigation yielded two thermal activation energies of 0.35 eV and 0.26 eV, respectively, for the traditional electroceramic processed samples. From the observed dielectric spectra it is concluded that the disappearance of the critical point behaviour in the E_B versus d plot is attributed to the improvement in the degree of uniformity of grain boundary distribution. Thus, the grain boundaries are also presumed to possess an enhanced degree of uniformity.