Combined spectrophotometry and computer based pH metric investigation on the mixed ligand complex formation equilibria of CuII with glycylglycine (H3N+CH2CONHCH 2COO-, HGG±) and guanylurea [H 21NC-(=0)2NHC(=3NH) 4NH2, HGu] in aqueous solution at 25 ±1 °C, at a fixed ionic strength, I = 0.1 mol dm-3 (NaNO3), indicate variety of binary and ternary complexes : [Cu(GG)+aq], [Cu(GG-H)(H2O)], [Cu(GG-H)(OH)-], [Cu(Gu)+aq], [Cu(Gu)(OH)], [Cu(Gu-H)(OH)-], [Cu(GG-H)(Gu)-], [Cu(GG-H)(Gu-H)2-] and [Cu(GG-H)(Gu-2H)3-]. At pH ≤ 4.5, GG- ion provides neutral (NH2, C=O) bidentate chelation to CuII and above pH 4.5-5, coordinated GG- is transformed to amide deprotonated glycylglycinate dianion, (GG-H)2-, which provides terdentate equatorial (NH2, N-, COO -) chelation to CuII. Consequently in the electronic spectra λmax of CuII is blue shifted. λmax of the ternary complex, [Cu(GG-H)(Gu)-], indicates a square pyramidal geometry existing, in a isomeric structural equilibrium, involving bidentate axial ( = 3N) and equatorial (O = C)/(H21N)Z(H1N = C) chelation by Gu- ion. Above pH 7, [Cu(GG-H)(Gu)-] undergoes deprotonation of the (H1N=) and HO-(enolic) moieties of coordinated Gu- in successive steps.