The U.S. Department of Energy has selected caustic-side solvent extraction as the preferred cesium removal technology for the treatment of high-level waste stored at the Savannah River Site. Data for the solubility of the extractant, calix[4]arene-bis（tert-octyl benzo-crown-6）, acquired and reported for the Salt Processing Program down-select decision, showed the original solvent composition to be supersaturated with respect to the extractant. Although solvent samples have been observed for approximay 1 year without any solids formation, work was completed to define a new solvent composition that was thermodynamically stable with respect to solids formation and to expand the operating temperature with respect to third-phase formation. Chemical and physical data as a function of solvent component concentrations were collected. The data included calix[4]arene-bis（tert-octyl benzo-crown-6） solubility; cesium distribution ratio under extraction, scrub, and strip conditions; flow sheet robustness; temperature
range of third-phase formation; dispersion numbers for the solvent against waste simulant, scrub and strip acids, and sodium hydroxide wash solutions; solvent density; viscosity; and surface and interfacial tension. These data were mapped against a set of predefined performance criteria. The composition of 0.007 M calix[4]arene-bis（tert-octyl benzo-crown-6）, 0.75 M 1-（2,2,3,3-tetrafluoropropoxy）-3-（4-secbutylphenoxy）- 2-propanol, and 0.003 M tri-n-octylamine in the diluent Isopar? L provided the best match between the measured properties and the performance criteria. Therefore, it is recommended as the new baseline solvent composition.