Positron-annihilation spectroscopy (PAS) was used in the lifetime mode to study changes in the grain-boundary defect equilibrium associated with dc-bias-induced degradation of a ZnO varistor. The PAS lifetime spectra were collected while the sample was under an applied bias ranging from 100 V ( ∼ 400 V/cm) to 500 V ( ∼ 2000 V/cm). The current through the sample was continuously monitored. The simple trapping model was used to interpret the lifetime PAS results and to obtain an estimate for the positron-capture rate. The experimental results show that an increase in the bias voltage results in a decrease in the positron-trap density and an increase in the positron lifetime associated with the dominant positron trap. These results are explained on the basis of a decrease in the concentration of negatively charged zinc vacancies at the grain boundary. The PAS results support the ion-migration model for degradation, which suggests that the bias-induced migration of positively charged zinc interstitials to the grain boundary reduces the concentration of negatively charged zinc vacancies at the boundary. This results in a reduction in the barrier height (degradation) and is consistent with the PAS data.