Department of Chemistry, University College of Science, 92 Acharya Prafnlla Chandra Road, Calcutta-700 009 Manuscript received 33 July 1990, revised 39 October 1991, accepted 6 December 1991 Combined pH-metric and spectrophotometric study on the complexation equilibria of CoII, NiII, Cull and ZnII with the model peptide ligand. salicyloylglycylglycine, (o)HO -C6H4CONHCH2CONHCH2COOH provided evidence of formation of only 1 : 1 metal : ligand complexes, M(H-2 L)2-. in which the metal ions are coordinated by the two deprotonated amide nitrogen atoms along with phenolic and carboxylic oxygen atoms of the ligand. Proton-ligand and metal-ligand constants were determined in 50% (v/v) ethanol-water medium at a fixed ionic strength (l=0.2 M NaNO3) at 30 + 0.1º. The complexation equilibria were worked out and correlated with the mode of bonding of the ligand.