The stability constants of ternary complexes of Co(II), Ni(II) and Cu(II) with L-Ornithine as primary ligand and Ethylenediamine as a secondary ligand were determined pH-metrically. The study was conducted in varying concentrations (0-60% v/v) of DMSO-water mixtures at an ionic strength of 0.16 mol L-1 using NaCl as an electrolyte at 303.0 K. The Value of Δ log K which is responsible for the extra stability of ternary complexes was discussed on the basis of statistical parameters and the nature of the species formed. The formation of various ternary species was established by modeling studies using the computer program MINIQUAD75. The formation and distribution of different species with relative concentrations of metal and ligands (M:L:X = 1.0:2.5:2.5, 1.0:2.5:5.0, 1.0:5.0:2.5) with varying pH were represented in the form of distribution diagrams drawn using HYSS HYPERQUAD. Influence of the solvent on the speciation was discussed based on the dielectric constant of the medium. The stability of the mixed ligand complexes are discussed in terms of the molecular structure of L– Orinthine and Ethylenediamine as well as the nature of the metal ion.