The solid-solution oxalates of the series CoxCu1-xC2O4.nH(2)O were prepared by coprecipitation from nitrate solutions. The characterization of the coprecipitates was carried out using X-ray powder diffraction (XRD), scanning electron microscopy and thermal analysis (TG/DTA) experiments in nitrogen. The results reveal the formation of a solid-solution with high cobalt content (x = 0.7) whereas other compositions with x = 0.3 or 0.5 did not form. The final solid products of the thermal treatment of the investigated oxalates are characterized by the formation of oxygen-deficient non-stoichiometric oxides. The thermal decomposition of the solid-solution and its mechanical mixture with the same mole ratio are very different. The first gave mixed lattice oxides in tri- and divalent states, whilst the latter gave separate oxides in the divalent state.