The electrical conductivity of γ-irradiated and unirradiated finely-divided spinels of composition (Zn x2+Fe 1−x2+Fe 23+)O 42−was studied in a nitrogen atmosphere as a function of temperature. The results of both γ-irradiated and unirradiated ferrites with compositions x ⩽ 0.79 showed that the electrical conduction occurred by fast electron exchange amongst Fe2+ and Fe3+ ions situated on octahedral sites in the spinel lattice, by a hopping mechanism. For composition x > 0.79 it was found that the transition of the charge carriers through cation vacancies is the predominant mechanism. Some breaks in the conductivity-temperature curves were found near the Curie points. Seebeck voltage measurements showed that γ-irradiated and unirradiated spinels behave as n-type semiconductors. The effect of γ-irradiation on the conductivity, activation energy, charge carriers and the conduction mechanism is discussed.