The　structure　and　stoichiometries　of　the　complexes　that　could　be　formed　between　Cu2+　and　3,4-dihydro-3-(2-oxo-2-phenylethylidene)-quinoxalin-2(1H)-one　(1)　were　investigated　by　various　spectral　techniques　such　as　IR,　fluorescence,　UV-vis　and　electron　paramagnetic　resonance.　The　results　suggest　that　initially　3:1　and　2:1　(1/Cu2+)　complexes　are　formed　at　low　Cu2+　concentration　and　upon　adding　more　Cu2+,　1:1　(preferred)　and　1:2　complexes　are　generated.　Since　1　possesses　two　possible　binding　sites,　further　exploration　was　done　by　testing　the　binding　ability　of　Cu2+　to　fragments　of　1,　namely　-enaminoketone　derivatives　(2-3)　and　quinoxaline-2-one　(4),　and　by　executing　calculations　of　thermodynamic　parameters　of　the　reaction　between　1　and　Cu2+　in　ethanol,　optimized　geometries　of　the　possible　complexes,　and　estimation　of　stability　constants　at　various　stoichiometries.　Consequently,　a　step-by-step　binding　mechanism　is　suggested　for　formation　of　various　complexes　between　1　and　Cu2+.