A model is proposed here to explain how the chemical features of metal oxide varistors can alter their nonohmic physical behavior, based on nonohmic similarities in the electrical properties of ZnO- and ${\mathrm{SnO}}_{\mathrm{2}}$ -based varistors. The proposed model explains the electrical properties of ZnO- and ${\mathrm{SnO}}_{\mathrm{2}}$ -based varistors before and after thermal treatments in oxygen- and nitrogen-rich atmospheres, which cause similar changes in the nonohmic feature of these polycrystalline ceramics with greatly differing chemical compositions and microstructures. The model is based on the key role that oxygen plays in varistor grain boundaries, independently of the type of ceramic system (ZnO-, ${\mathrm{SnO}}_{\mathrm{2}}$ - or even ${\mathrm{SrTiO}}_{\mathrm{3}}$ -based varistors) involved.