Structures and stabilization due to stacking of ternary copper(II) complexes containing an aromatic amino acid (AA) and an aromatic diamine (DA), Cu(AA)(DA), have been investigated by potentiometric, spectroscopic, and X-ray diffraction methods. For the systems with AA para-X-substituted L-phenylalanine (L-XPhe; X = H, NO2, OH, NH2) and DA = 2,2’-bipyridine (bpy) or 1,10-phenanthroline (phen), the difference absorption spectra in the region 320-400 nm exhibited a peak assignable to the charge transfer interaction between the aromatic rings of DA and L-XPhe, the intensity being in the order NH2 > OH > H much greater than NO2 With respect to X. The stability constants of Cu(AA)(DA) determined for AA = DL-XPhe (X = F, Cl, Br) and L-XPhe (X = NH2, NO2, I) at 25 degrees C and I = 0.1 M (KNO3) indicated that stabilization of Cu(L-XPhe)(DA) relative to Cu(L-Ala)(en) (Ala = alanine; en ethylenediamine) is in the order Br > OH > Cl approximate to NH2 > NO2 greater than or equal to H greater than or equal to F. The structures of [Cu(L-NH2-Phe)(bpy)]NO3 . H2O (1), [Cu(L-Tyr)(phen)]ClO4 . 2.5H(2)O (2), [Cu(L-Phe)(phen)]Cl . 3H(2)O (3), and [Cu(L-Phe)(bpy)]ClO4 . H2O (4), isolated as crystals, were determined by the X-ray diffraction method : 1, orthorhombic, P2(1)2(1)2(1), a = 10.292(1) Angstrom, b = 13.576(4) Angstrom, c = 14.407(1) Angstrom, V = 2012.9 Angstrom(3), Z = 4, R = 0.037, R(w) = 0.038; 2, orthorhombic, P2(1)2(1)2(1), a = 18.20(2) Angstrom, b = 32.63(1) Angstrom, c = 8.14(1) Angstrom, V = 4833 Angstrom(3), Z = 4, R = 0.111, R(w) = 0.087; 3, monoclinic, P2(1), a = 11.738(2) Angstrom, b = 16.301(1) Angstrom, c = 11.795(1) Angstrom, beta = 102.01(1)degrees, V = 2207.4 Angstrom(3); Z = 2, R = 0.045, R(w) = 0.035; 4, monoclinic, P2(1), a = 9.954(2) Angstrom, b = 24.179(3) Angstrom, c = 9.780(2) Angstrom, beta = 107.32(1)degrees, V = 2257.1 Angstrom(3), Z = 4, R = 0.065, R(w) = 0.060. All of the complexes have a similar distorted square-pyramidal structure around the central Cu(II) ion. While 1 and 2 have a structure involving aromatic ring stacking in the solid state, 4 has a structure without it and 3 has both types of structures in the unit cell.