The results are given of an investigation into the effect of artificial aging for 400 hr at 250 °C on the structure and properties of magnesium-manganese-zinc ferrite of composition Mg0.406Mn0.633Zn0.272Fe1.689O3.984. The structure was investigated by x-ray structural and x-ray spectral analysis, neutron diffraction, and electron microscopy. The results of the structural investigations were compared with magnetic, electric and SHF properties. It was shown that the changes found in the structure and properties are related to a change in the degree of inversion of the spinel structure and the valency of the manganese ions.