Cooling rate effects on ZnO-Bi2O3 varistors are presented. In particular, the oxygen vacancies (V o + ) concentration was determined by electron paramagnetic resonance (EPR) spectroscopy and related to the conductivity. It was found that a higher cooling rate provokes an increase of the V o + concentration and a higher conductivity. Current density versus temperature curves are fitted with a modified Schottky barrier model which reflects a higher active donor concentration near the grain interface.