The Verwey transition is analyzed by measuring the initial permeability as a function of tempera‐ture in polycrystalline CoxFe3‐xO4 (with 0 ≦ x ≦ 0.04). The transition temperature slightly decreases with cobalt concentration. A small impurity concentration (≦ 5%) is sufficient to wipe out any sign of the transition. Most samples show a marked thermal hysteresis between heating and cooling curves above the transition.