Speciation of binary complexes of Pb(II), Cd(II) and Hg(II) with L-3,4-DIhydroxyphenylalanine (dopa) in (0-60% v/v) acetonitrile-water mixtures have been studied pH-metrically at a tempature of 303 K at an ionic strength of 0.16mol L^-1. The models for binary species contain ML₂H for Pb(II), MLH, ML₂H for Cd(II) and MLH, ML₂H₂ for Hg(II). The trend in the variation of stability constants with the mole fraction of acotonitrile was explained on the basis of electrostatic and non electrostatic forces. Distribution of the species with pH at different compositions of acetonitrile-water media was also presented.