The time decrease of initial permeability has been measured for several stoichiometric and nonstoichiometric compositions of MnxFe 3-xO4+y. The samples were in the form of polycrystalline toroids with compositions x =0.5, 0.7, 0.85, and 0.9. The stoichiometric compositions, y = 0, were fired to maintain a metaloxygen ratio of 3:4, while the nonstoichiometric samples were fired to produce the maximum vacancy concentration. The permeabilities were measured at a peak driving field of 0.002 oe in the temperature range -15°to +35°C. The time decrease data was described by three relaxation times. Activation energies calculated on the basis of these relaxation times were within experimental error equal and independent of the Mn/Fe ratio, i.e., Q = 0.46±0.06 ev. The activation energy for the nonstoichiometric samples was equal to that observed for the stoichiometric samples. The magnitude of the effect in the stoichiometric samples was found to decrease approximately linearly with the increasing Mn/Fe ratio. © 1960 The American Institute of Physics.