The synthesis of the first example of a dinuclear syn-anti carboxylate-bridged copper(II) complex from a related hydrogen-bonded dinuclear copper(II) complex with the ligand 1,3-dimethyl-5-[(2-carboxyphenyl)azo]barbituric acid by thermal treatment is discussed. This method is particularly useful because direct synthesis of the dinuclear syn-anti carboxylate-bridged copper(II) complex is difficult. Both complexes have been studied by thermogravimetric and differential scanning calorimetric techniques, and from spectroscopic and X-ray diffraction measurements.