Because both manganese and iron assume multiple valences in oxide structures, it is possible to fabricate spinels over a continuous range of composition from Mn3O4to Fe3O4. Magnetic‐moment data for samples fabricated within this range are reported and an interpretation of the data in terms of the distribution of ions of manganese and iron is presented.The data agree with previous indications that manganese ferrite, MnFe2O4, is an almost completely normal spinel with Mn2+onAsites and Fe3+onBsites. In addition, the usual assumption that excess iron replaces manganese as Fe2+onBsites is substantiated. However, the data are not compatible with the previously held assumption that Fe3+is replaced by excess manganese as Mn3+. Instead, the variation of magnetic moments and Curie temperatures with composition is interpreted in terms of a model in which excess manganese replaces Fe3+as Mn4+and an equal amount of iron is converted to Fe2+. When the manganese content gets so high that all the iron is Fe2+(Mn2FeO4), manganese then replaces the iron as Mn3+and reconverts an equal amount of Mn4+to Mn3+, and the Mn3+, because of its tetragonal nature, introduces spinel twinning.It is proposed that the Mn4+&sngbnd;Fe2+pair in octahedral coordination is stabilized with respect to a Mn3+&sngbnd;Fe3+pair because of the interaction of the manganese orbitals with the 2porbitals of the surrounding oxygen anions.