The formation of an aqueous ternary complex was investigated in the malonate-based Advanced TALSPEAK system by solvent extraction and optical spectroscopy. Features of the spectroscopy confirmed the presence of a ternary complex between malonate, HEDTA, and the trivalent americium or lanthanide ion. The ternary complex was then incorporated into a solvent extraction model to refine stability constants under Advanced TALSPEAK conditions. Good agreement was found in the equilibrium constants determined by both methods. These values were then used to model metal speciation and predict extraction behavior at different concentrations of malonate and HEDTA. Calculated metal distribution ratios show good correlation with experimental values. The apparent simplicity of the chemical interactions and predictability of malonate-buffered Advanced TALSPEAK provides a significant advantage over the less ideal interactions of conventional TALSPEAK.