[en] ⁵⁷Fe doped CuO, La_2-xSr_xCuO₄ and MnO₂ have been prepared by the Nitrate route. In all the three oxides the isomer shift values lie in the range 0.25-0.36 mm/s which is typical for the Fe³⁺ state. The Moessbauer parameters of iron doped CuO differ considerably from those obtained in earlier source experiments. The difference may be attributed to the aftereffects of nuclear decay. (orig.)