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The conduction mechanisms in nickel‐doped yttrium aluminum garnet (Ni:YAG) have been studied as a function of temperature and partial pressue of oxygen. ac conductivity and ionic transference measurements show that Ni:YAG is a mixed ionic‐electronic conductor with an ionic mobility characterized by an activation energy of 2.0–2.2 eV. The reduction of Ni+3 to Ni+2 causes an increase in the oxygen vacancy concentration and a concurrent rise in the magnitude of the ionic conductivity. Codoping with zirconium, on the other hand, fixes the nickel in the divalent state, increases the n‐type conductivity, and lowers the degree of ionic conductivity. A defect model is presented which is consistent with all of these observations.