Liquid-liquid extraction experiments, UV/vis spectra, and equilibrium modeling by use of the program SXLSQA have been employed to reveal the origin of synergism in the extraction of CU(ll) from sulfuric acid by the tetradentate macrocycle tetrathia-14-crown-4 (TT14C4) combined with the cation exchanger didodecylnaphthalene sulfonic acid (HDDNS) in toluene. The key feature of the system is the 1.1 complexation of Cu(ll) by TT14C4. HDDNS functions both as a source of exchangeable protons and as a solvating agent that promotes the formation of small, hydrated aggregates. In the absence of the cation-exchange vehicle provided by HDDNS, the macrocycle does not extract Cu(ll) detectably. Combined with HDDNS, however, TT14C4 significantly enhances the extraction of the metal by HDDNS. Comparative UV/vis spectrophotometry indicates the formation of the deep-blue chromophore attributed to the complex ion Cu(TT14C4)(2+), wherein the planar set of four endo sulfur donor atoms of TT14C4 circumscribes the metal cation. Modeling of the extraction and spectral absorbance data by use of the program SXLSQA supports the conclusion that this 1.1 complex accounts for essentially ail of the extracted Cu(ll) in excess of that extracted by HDDNS alone. The best model includes HDDNS aggregates incorporating Cu(TT14C4)(2+)) or Cu2+ ions. In the analysis, activity effects in both aqueous and organic phases have been taken into account by the Fitter and Hildebrand-Scott treatments, respectively, with explicit inclusion of the formation of aqueous hydrogen sulfate ion. The model accounts for the effect of variation of the solute components CuSO4, H2SO4, HDDNS, and TT14C4.