The decomposition of the freeze dried Cu(II)-Ni(II)-Fe(III) formate precursors at 1000 degrees C in air yields complex oxides CuxNi1-xFe2O4 +/-delta (0 <= x <= 1) with a cubic spinel structure. For x < 0.7, single phase spinels are formed at 1000 degrees C. However, for 0.7 <= x <= 1, Copper oxide (CuO) is identified as a second phase and the formation of a pure spinel phase requires an increase of the iron content in the mixture. For example, Cu0.81Ni0.1Fe2.09O4 is a single phase at 1000 degrees C/air. Other single spinel phases Cu0.5+yNi0.5-y-zFe2+zO4 +/-delta, 0 <= (y + z) <= 0.5, in the phase triangle Cu0.5Ni0.5Fe2O4-CuFe2O4-Cu0.5Fe2.5O4 have been synthesized under special p(O-2)/T-synthesis conditions. The increase of the iron content requires an increase of the reaction temperature and/or a decrease of the p(O-2) in the reaction gas stream. The oxygen exchange between Cu0.9Fe2.1O4.02 and the reducing gaseous phases shows that the non stoichiometry delta of copper ferrite is only about +/- 0.03. Significant changes in the oxygen content lead to the separation in different phases. The electrical and magnetic properties of copper ferrite samples depend on their chemical composition and preparation conditions. (c) 2006 Springer Science+ Business Media, Inc.