School of Chemistry, Andhra University, Visakhapatnam-530 003, Andhra Pradesh, Ind1a E-mail : gollapallinr@yahoo.com Manuscript received 09 February 2010, revised 02 July 2010, accepted 30 July 2010 Complexation between essential metal ions and 1,10-phenanthrolinc was studied by monitoring hydrogen ion concentration pH metrically at 303 K and at an ionic strength of 0.16 mol dm-3. The existence of various binary species was established from modeling studies using the computer program MINIQUAD75. The best lit chemical models were selected based on the statistical parameters like crystallographic 'R' factor and .,um of squares of residuals in mass-balance equations. The formation and distribution of different species with relative concentrations (M : L = 1 :2.5, 1 : 3.75 and 1 : 5) of metals and ligand, with varying pH was represented in the form of distribution diagrams. Influence of the solvent on the speciation was discussed based on the dielectric constant of the medium.