Abstract: The stability constants of ternary complexes of Co(Ⅱ) with iminodiacctic acid (IDA) as the primary ligand and glycine (Gly),Alaninc (Ala), Nor-Valine (Nor-Val),phenyl-alanine (Phc),methionine (Met),Serine (Ser),asparagine (Asn) as the secondary ligand have been determined.Auxiliary data,such as the acid dissociation constants and the binary stability constants of Co(Ⅱ)-IDA and Co(Ⅱ)-Met,have also been obtained under identical conditions. The stability of the ternary complexes and the coordination manner of some potential tridcntate ligands have been discussed in terms of their stability-constant data.