Mössbauer spectra and X-ray powder diagrams of synthetic 57Co and 57Fe oxides having the two lattice-site spinel structure have been obtained. The distribution of iron and cobalt atoms between the octahedral and tetrahedral lattice sites has been determined by means of isomer shift and quadrupole splitting measurements. A new mechanism is proposed in order to explain the distribution of 57Fe as well as its charge state when it enters the lattice as an impurity. At high temperature a mixed valence structure of Co 3O 4 was found. It is argued that the charge state of the daughter iron depends on the vacancy concentration in the environment of those sites where the corresponding decay takes place. Single line sources are obtained under well determined conditions.