Department of Chemistry. Osmania University, Hyderabad-500 007 Manuscript received 12 September 1994. revised 31 July 1995. accepted 8 August 1995 Complex equilibrium studies on some aminomethylynephosphonic acids like N-phosphonomethyliminodiacetic acid (NPMI) and N· phosphonomethylglycine (NPMG) for the determination of stability constants with a few bivalent metal ions, viz. Be, Zn and Cd have been carried out in an aqueous medium at 0.10 M (KNO3) ionic strength and 20, 30, 40 and 50º by employing pH-metric titration techniques. NPMI forms stronger metal thelates than NPMG because of its higher basicity. The Ben complexes are more stable over those of the ZnII and Cdll complexes. Mixed ligand complexes involve some N-O, O-O, N-N and N-O-O donors like glycine, β-alanine, catechol, 5-sulpho.alicylic acid, oxalic acid, 1,10-phenanthroline. 2,2'-hipyridyl and iminodiacetic acid. Stepwise and overall stability constants. thermodynamic parameters, differences in stabilities, percentages of relative stabilisation values and percentage of distribution of various metal chelated species have been evaluated and discussed in terms of basicity, denticity and statistical aspects of the ligands.