High non-linear J×E electrical characteristic (α=41) were obtained in the Nb₂O₅ and Cr₂O₃ doped CoO highly densified SnO₂ ceramics. X-ray diffraction analysis showed that these ceramics are apparently single phase. Electrical properties and microstructure are highly dependent on the Cr₂O₃ concentration and on the sintering temperature. Excess of Cr₂O₃ leads to porous ceramics destroying the material’s electrical characteristics probably due to precipitation of second phase of CoCr₂O₄. Dopant segregation and/or solid solution formation at the grain boundaries can be responsible for the formation of the electrical barriers which originate the varistor behaviour.