Freeze-dried complex oxalates of copper and niobium are reactive precursors for ternary copper niobium oxides. The thermal decomposition of the amorphous precursors has been investigated by DTA, TG and mass spectroscopy. The gaseous decomposition products are assigned to steps in the TG graph. Above 500 degrees C, crystallization of complex oxides takes place. In the temperature range up to 900 degrees C, only the ternary phases CuNb2O6 and Cu3Nb2O8 can be synthesized. The thermal decomposition of the oxalate precursor is advantageous with regard to the preparation of pure monoclinic CuNb2O6. The transformation between monoclinic and orthorhombic CuNb2O6 is reversible with a high activation energy of transformation.