Chemical speciation of Maleic acid complexes of Pb(II) and Cd(II) in the presence of (0-50% v/v) Dimethyl sulfoxide-water mixtures has been studied pH-metrically at 303.0K and at an ionic strength of 0.16 molL-1. The models containing different number of species were refined by using the computer program MINIQUAD75. The number of species in the models is chosen based on exhaustive modeling. The predominant species detected are ML2, ML2H and ML3. The best fit chemical models were arrived at based on statistical parameters. The appropriateness of the models is ascertained by studying the effect of errors in concentrations of ingredients. The trend in variation of stability constants with change in the composition of medium is explained on the basis of electrostatic and non-electrostatic forces. Chemical speciation was discussed based on the distribution diagrams.