The stability constants and coordination modes of the copper(II) heteroligand complexes Cu 2+ –L 1 –L 2 formed by ethylenediamine (en), diethylenetriamine (dien), N,N,N′,N″,N″-pentamethyldiethylene triamine (Me 5 dien) (=ligand L 1 ) and DL-2-amino-3-phosphonopropionic acid (β-Asp(P)=ligand L 2 ) have been determined in an aqueous solution (25.00°C and I=0.2moldm −3 KCl) by pH-metric, spectrophotometric and EPR methods. Heteroligand complexes with [Cu(L 1 )(β-Asp(P))] − stoichiometry are formed in all studied Cu 2+ –L 1 –L 2 ternary systems. Additionally, in the system with en and dien the protonated [Cu(L 1 )(H–β-Asp(P))] species is postulated. The results indicate the tridentate coordination mode of β-Asp(P) and the tetragonal geometry for the [Cu(en)(β-Asp(P))] − species. If ligand L 1 is dien or Me 5 dien, five-coordinated heteroligand copper(II) complexes are supported. For the [Cu(dien)(β-Asp(P))] − complex the geometry slightly deviated from square pyramidal is postulated, whereas for the [Cu(Me 5 dien)(β-Asp(P))] − species somewhat stronger geometry distortion is observed. In both complexes of tridentate amines, β-Asp(P) reveals similar coordination ability as α-Ala and coordinates to the copper(II) ion in the amino acid manner with the formation of five-membered chelate ring.