Electrical conductivity and emf measurements on polycrystalline , as functions of the partial pressure of oxygen, at temperatures ranging between 800° and 1100°C, are used to study some transport and thermodynamic properties of nearly stoichiometric and nonstoichiometric . Due to the localized character of electrons in ionic, hopping‐type semiconductors, results are discussed with a chemical approach, in terms of the dependence of the activation enthalpy for the electrical conductivity and of the activity coefficients of Ni3+ on the partial pressure of oxygen. A defect model for nonstoichiometric is given, which involves the presence of defect clusters (microdomains) with an inverse spinel structure. Furthermore, electrical conductivity results in nearly stoichiometric are presented and discussed, which show straightforward evidence of the influence of iron doping in the range 100–500 ppm on the “intrinsic” character of the electrical conductivity.