BACKGROUND: Di-(2-ethylhexyl)phosphoric acid (D2EHPA, H(2)A(2)) has been used extensively in hydrometallurgy for the extraction of rare earths, but it has some limitations. Synergistic extraction has attracted much attention because of its enhanced extractabilities and selectivities. In the present study, sec-octylphenoxyacetic acid [CA12, H2B2) was added into D2EHPA systems for the extraction and separation of rare earths. The extraction mechanism of lanthanum with the mixtures and the separation of lanthanoids and yttrium were investigated. RESULTS: The synergistic enhancement coefficient for La3+ extracted with D2EHPA+CA12 was calculated as 3.63. La3+ was extracted as La(NO3)(2)H(2)A(2)B with the mixture. The logarithm of the-equilibrium constant was determined as 0.80. The thermodynamic functions, Delta H, Delta G, and Delta S were calculated to be 4.03 kJ mol(-1), -1.96 kJ mol(-1), and 20.46 J mol(-1) K-1, respectively. The mixtures have synergistic effects on Ce3+, Nd3+, and Y3+, with an especially strong synergistic effect on Y3+. Neither synergistic nor antagonistic effects on Dy3+ and weak antagonistic effects on Lu3+ were found. CONCLUSION: Mixtures of D2EHPA and CA12 exhibit evident synergistic effects when used to extract La3+ from nitric solution. The stoichiometries of the extracted complexes have been determined by graphical and numerical methods to be La(NO3)(2)H(2)A(2)B with the mixture. The extraction is an endothermic process. The mixture exhibits different extraction effects on rare earths, which provides possibilities for the separation of Y3+ from Ln(3+) at a proper ratio of D2EHPA and CA12. (c) 2008 Society of Chemical Industry