The formation equilibria of complexes of Pb (II), Cd (II), and Hg (II) with 1,10-Phenanthroline were investigated pH metrically in Dimethyl formamide-water mixtures (0-60%v/v) at 303 K and ionic strength of 0.16 mol L^-1. The dominant species detected were ML and ML₃ for Pb (II), Cd (II) and Hg (II). The appropriateness of the experimental conditions were verified by introducing errors deliberately. The models containing different numbers of species were refined by using the computer program MINIQUAD75. Selection of the best fit models was based on the statistical parameters. The trend in variation of stability constants of the complexes with dielectric constant of the medium was attributed for the formation and possible structures of the complex species presented.