The species and equilibria present in the synergistic extraction system for strontium, composed of didodecylnaphthalene sulfonic acid (HDDNS) and dicyclohexano-18-crown-6 (DC18C6) in CCI4, have been studied by IR and proton NMR spectroscopies, vapor pressure osmometry (VPO), and Karl Fischer (KF) titrations. With the aid of least-squares computer methods to fit the IR and VPO results, a series of proposed equilibria and associated equilibrium constants have been shown to adequately account for all of the observed behavior. The sulfonic acid and the salt of the acid alone in CCI4solution form the micellar species (HDDNS·xH2O)11(× is in the range 7-8) and (Sr(DDNS)2· 11 H2O)6respectively. The sulfonic acid alone, under conditions of intermediate loadings with Sr2+, yields the predominant species (Sr(DDNS)2)3(HDDNS)5·×H2O. When alone in solution, DC18C6 is a monomer, but it forms adducts with the free sulfonic acid and its strontium salt, yielding the respective product species (HDDNS)7·DC18C6· ×H2O and Sr(DC18C6)(DDNS)2· 2H2O. An excess of HDDNS or Sr(DDNS)2in the presence of Sr(DC18C6)(DDNS)22H2O yields predominantly Sr(DC18C6)(DDNS)2(HDDNS)2· ×H2O and (Sr(DC18C6)(DDNS)2)2(Sr(DDNS)2)33' · ×H2O respectively.