A combination of electrical and ceramic microstructural data has been used to determine the voltage drop across the ZnO-intergranular barrier in some simple and multicomponent ZnO-based varistors at the onset of high non-Ohmicity. The data are obtained by measuring the varistor breakdown field (V/mm) and average grain size of the conductive ZnO matrix systematically as functions of sintering temperature and time. Results show that the voltage drop in ZnO varistors is equal to the sum of the voltage drops vb across the ZnO-intergranular barriers. In the case of GE-MOVR varistors, vb is determined to be 2.3 V per barrier.