Theoretical considerations of metal complex formation in aqueous solutions were used to develop a computer program (CHELATE) to cakulate all equilibrium species (free metal ions, metal complexes, etc.) in any user-defined system, such as xylem fluid. Mass-balance equations were established to describe each free metal ion and each free ligand concentration as a function of solution pH, total metal or total ligand, hydrogen-association constants, and the stability constants of known metal complexes. A default data base can be altered by the user to define any desired system covered by the stored equilibrium data. The program can currently handle nine metal ions, 35 ligands, and 500 complex species. The validity of the program was confirmed by using experimental test systems in which free-metal ion activity measurements were made with ion-selective electrodes. Program CHELATE was used to calculate the distribution of six metals in 0-to 1-hour exudate from soybean (Glycine max L. Meff.) and tomato (Lycopersicon escuentum Mill.) plants grown in normal and Zn-phytotoxic nutrient solutions. The results indicated that Fe is bound by citric acid, and Cu is bound by several amino acids in the normal-Zn exudate. Most of the Cu in soybean exudate is bound to asparagine and histidine. In tomato, Cu is bound to histidine, glutamine, and asparagine. Zinc, Mn, Ca, and Mg are bound primarily by citric acid and malic acid in both species; the per cent bound for these metals is lower than that for Fe and Cu. Zinc phytotoxicity caused equilibrium concentration shifts and resulted in the formation of several adtional metal complexes not found in the normal-Zn exudate. Work reported in a companion paper (24) showed that xylem ' This work represents cooperative research from the fluid contains many natural complexing compounds and indicates that metal ions should form metal complexes in vivo in the xylem sap. There is in vitro electrophoretic evidence for stable metal complex formation in xylem fluid (2, 20, 23) but no quantitative estimates of the equilibrium distribution of metals, ligands, metal complexes, and other solutes. In a solution containing several metals and ligands, simultaneous competing equilibria distribute the metals among the ligands, creating numerous complexes. Such a system can be described both qualitatively and quantitatively; however, quantification of all equilibrium species in a multimetal, multiligand system is far more difficult than listing the theoretically permissible (qualitative) species. Although many aqueous species cannot be measured, equilibrium chemistry can be used to estimate metal binding in aqueous solutions. The degree to which hydrated metal ions associate with various ligands depends primarily on the relative metal and ligand concentrations , solution pH, and the pertinent empirically determined stability constants. These variables have been used to determine equilibrium distributions of metals among ligands in several diverse natural aqueous solutions (6, 9, 21). Most of these systems are...