This paper assesses the correlation between impedance and reference voltage of degraded metal oxide varistors. Low-voltage varistor devices are artificially degraded using switching impulse currents and thermal stresses. Impedance and reference voltage are measured before and after degradation tests. Linear regression is applied in order to model the relationship between observed changes in varistor impedance and reference voltage. The validity of the obtained model is subsequently verified using analysis of variance. Results show no linear relationship between varistor impedance decrease and reference voltage.