Department of Inorganic & Analytical Chemistry, Andhra University, Visakhapatnam-530 003, Andhra Pradesh, India E-mail : gollapallinr@yahoo.com Manuscript received online 07 December 2012, revised 24 January 2013, accepted 01 February 2013 An electrometric investigation was made on the formation equilibria of binary complexes of aspartic acid with CaII, MgII and ZnII at 303 K and an ionic strength of 0.16 mol L–1. The stability constants of the species ML, ML2H2, ML2H3, ML2H4 were refined with the computer program MINIQUAD75. The stability constants of ZnII complexes were found to be greater than those of CaII and MgII due to pseudo inert gas configuration of ZnII. The formation and distribution of different species were represented in the form of distribution diagrams. Influence of the co-solvent on the speciation was discussed based on the dielectric constant of the medium.