Chemical speciation of binary complexes of Co(II), Ni(II), and Cu(II) ions with maleic acid have been studied pH metrically in the concentration range of 0.0-2.5% v /v nonionic surfactant(TX100) – water mixtures main taining an ionic strength of 0.16 M at 303K. Alkalimetric titr ations were carried out in different relative conce ntrations of metal and maleic acid. Models containing different numbers of species were refined by using the comput er program MINIQUAD75. Selection of the best fit chemical mode ls was based on statistical parameters and residual analysis. The predominant complexes formed are ML, ML 2 and ML 2 H 2 for Co(II), Ni(II), and Cu(II). The trend in the variation of stability constants with change in mol e fraction of the medium was explained on the basis of electrostatic and non-electrostatic forces. Distrib utions of species, formation equilibria and effect of influential parameters on the stability constants have been pre sented. The possible structures of the various spec ies are elucidated on the basis of the analysis of the pH m etric data.