Speciation of Pb(II), Cd(II) and Hg(II) complexes of 1,10-phenanthroline has been studied pH metrically in 1,4 dioxan–water mixtures (0.0–60.0 %, v/v) at 303 K and an ionic strength of 0.16 mol dm−3. The predominant species detected are ML and ML2 for Pb(II), ML, ML2 and ML3 for Cd(II) and ML2 and ML3 for Hg(II). The trend in the variation of stability constants of these species with decreasing dielectric constant of the medium is attributed to the electrostatic and non-electrostatic forces and the complexing ability of the co-solvent. The species distribution and the feasible equilibria for the formation of the species are also presented.