Cobalt ferrites, Co3−xFexO4, particularly forxabout or less than 2 are difficultly prepared as single crystal from the melt because they dissociate except at high partial pressures of oxygen. A two‐step procedure is described for obtaining single‐crystal cobalt ferrites from the melt at gas pressures of an atmosphere. The steps involve growth of a two‐phase ``single crystal''—possible because of the congruency of the oxygen lattice of the coexisting oxides. The two‐phase crystal is converted to the single‐phase crystal by appropriate oxidation under conditions in which diffusion is rapid.