A semi-empirical model of Suwannee River fulvic acid based on experimental evidence has been used as a basis for determining the structural characteristics of divalent metal ion binding. Three low-energy configurations of fulvic acid which contain differing orientations of various functional groups are reported. The structures of 1:1 and 2:1 metal ion - fulvic acid complexes have been determined for Cd2+, Pb2+, Mg2+, and Zn2+ bound to pairs of carboxylate groups in the three low energy configurations of fulvic acid. Standard enthalpies of formation are shown to be useful as indicators of metal ion - fulvic acid complex formation. A major characteristic of the phthalate model of Suwannee River fulvic acid is the locational flexibility of the carboxylic acid groups which accommodate ions of differing ionic radii. © 1998 Elsevier Science B.V.