Department of Chemistry, University College of Science, University of Calcutta, 92, Acharya Prafulla Chandra Road, Kolkata-700 009, India E-mail: mukherg@rediffmail.com Fax : 91-033-2351-9755/2241-3222 Manuscript received 11 January 2010, revised 24 February 2010, accepted 25 February 2010 Combined spectrophotometric and computer-based pH-metric investigations on the mixed-ligand complex formation equilibria or CuII with histidine [(C3H3N2)CH2CH(NH2)COOH, hisH\( \pm\)] and biguanide [NH2C- (=NH)NHC(=NH)NH2, Bg] in aqueous solution at a constant ionic strength (I= 0.1 mol dm-3) and 25 ± 1ºC, provided evidence of binary [Cu(hism)+], [Cu(hism-H)], (Cu(hlsm)2], (Cu(hism)(hism-H)-], [Cu(bism-H)22- ]and (Cu(Bg)2+] and ternary [Cu(bism)(Bg)+], [Cu(hism-H)(Bg)], [Cu(Bg)(OH)+] and [Cu(Bg-H)(OH)] complexes. Histidine provides both histamine-like ((NH2, N-imidazole i.e. (hism-)] and mixed histamine-like glycine-like ((NH2, N-imidazole) (NH2, COO-) i.e. (hism-)(gly-)] chelation, barring simple glycine-like (gly-) chelation. Binding affinity of CuII for (NH2, =NH) bidentate chelating biguanide is so strong that the histidine-imidazole deprotonated mixed-ligand complex, (Cu(hism-H)(Bg)] decomposes at pH > 10 to biguanide ( =NH) deprotonated ternary hydroxo complex, [Cu(Bg-H(OH)], setting free the his- ligand ion in solution, a phenomenon of great biological significance, in regard to medicinal applications of the title ligands and their metal derivatives.