Chemical speciation of binary complexes of Co(II), Ni(II) and Cu(II) with L-ornithine was studied pH-metrically in the concentrations range of 0-60% v/v DMSO-water mixtures maintaining an ionic strength of 0.16 mol L-1 at 303K. Alkalimetric titrations were carried out in different relative concentrations of metal and L-ornithine. Stability constants of various models of binary complexes were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The species detected are ML, ML2, MLH and ML2H2 for of Co(II), Ni(II) and Cu(II). The chemical speciation, metal bioavailability and transportation are explained based on the distribution diagrams drawn using HYSS HYPERQUAD.