Two copper(II) complexes of 2,3-pyrazinedicarboxylate (pzdca) with N,N'-dimethylethylenediamine (dmen), [Cu(2)(pzdca)(2)(H(2)O)(2)(dmen)(2)] center dot 6H(2)O (1) and 1,10-phenanthroline (phen), [Cu(pzdca)(phen)(2)] center dot 5.5H(2)O (2), have been prepared and characterized by elemental analyses, IR, UV/vis, magnetic measurement and single crystal X-ray diffraction. The complexes crystallize in the monoclinic space group P2(1)/c and triclinic space group P (1) over bar, respectively. In both complexes the copper coordinates pyrazine nitrogen and carboxylate oxygen as a bidentate ligand. The coordination sphere around Cu(II) is completed by two N atoms from dmen and four N atoms from phen groups and one axial position occupied by the carboxyl O atom from the symmetry related molecule in 1. The coordination sphere should be described as a distorted octahedral and (5 + 1)-geometry in 1 and distorted octahedral geometry in 2. While 1 shows a first dimeric arrangement, 2 is a monomer. The thermal analyses show that 1 and 2 decompose completely in four and five step thermal processes at 20-600C temperature intervals.