Proper choice of the starting compounds for the additive oxides enhances the non-linearity coefficient (α) to 40–60. High α-values can be achieved by using Bi 2O 3 containing Bi 5+ and consequently higher oxygen content. Bi 2O 3prepared by various low temperature chemical routes are more effective in increasing the α values. Ceramics formulated from ZnO + oxygen — excess Bi 2O 3 have α = 6 to 14, whereas in presence of the transition metal oxides, the same formulations exhibit α-values up to 60, with the sharp turn-on point on the current-voltage curves. The oxygen-excess behaviour of Bi 2O 3 is related to its defect-fluorite structure, with the excess oxygen located at the vacant tetrahedral voids and are charge compensated by Bi 5+ ions. This is supported by X-ray diffraction, optical reflectance spectra and electron paramagnetic resonance data. The depletion layer is formed at the pre-sintering stage as a result of electron trapping by oxygen evolved from the additives being chemisorbed on ZnO particles. The depletion layer is stabilized on either sides of the grain boundary by the higher valent transition metal ions that are preserved through charge compensation by cation vacancies, after the sintering.