The structure of the complex Fe-Co mixed oxides system, prepared as an industrial ammonia catalyst, is studied by TEM (transmission electron microscopy). The introduction of Co in the system create a high density of structural extended defects. Concentration of Co bigger than 10 a. % lead to the formation of a high density of precipitates which were identified as Co3O4in the magnetite matrix. The data in this phase identification was obtained by moiré fringe analysis.