A semi-empirical model of Suwannee River fulvic acid, based on spectroscopic and electrochemical evidence, has been used as a basis for determining the structural characteristics and computed enthalpies and entropies of both anhydrous and hydrated metal ion-fulvic acid complexes containing either magnesium, zinc, lead or cadmium. Anhydrous and hydrated versions of metal ion-phthalic acid complexes are used to model the phthalate binding site in fulvic acid. The hydrated metal ion-phthalic acid complexes with the lowest enthalpies of formation are those with carbonyl carbons pointing in the same direction relative to the benzene ring (cis). This result is also observed for the phthalate binding site in hydrated metal ion-fulvic acid complexes. The entropies of complex formation for both the anhydrous and hydrated metal ion-fulvic acid complexes are similar within each type of di-substitutcd metal ion-fulvic acid complex. Standard enthalpies of formation are an indicator of the relative strength of metal ion binding for both metal ion-phthalic acid and metal ion-fulvic acid complexes. © 1998 Elsevier Science B.V.