BACKGROUND: 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (HEHEHP, H(2)A(2)) has been applied extensively to the extraction of rare earths. However, there are some limitations to its further utilization and the synergistic extraction of rare earths with mixtures of HEHEHP and another extractant has attracted much attention. Organic carboxylic acids are also a type of extractant employed for the extraction of rare earths, e.g. naphthenic acid has been widely used to separate yttrium from rare earths. Compared with naphthenic acid, sec-nonylphenoxy acetic acid (CA100, H2B2) has many advantages such as stable composition, low solubility, and strong acidity in the aqueous phase. In the present study, the extraction of rare earths with mixtures of HEHEHP and CA100 has been investigated. The separation of the rare earth elements is also studied. RESULTS: The synergistic enhancement coefficient decreases with increasing atomic number of the lanthanoid. A significant synergistic effect is found for the extraction of La3+ as the complex LaH(2)ClA(2)B(2) with mixtures of HEHEHP and CA100. The equilibrium constant and thermodynamic functions obtained from the experimental results are 10(-0.92) (K-AB), 13.23 kJ mol(-1) (Delta H), 5.25 kJ mol(-1) (Delta G), and 26.75 J mol(-1) K-1 (Delta S), respectively. CONCLUSION: Graphical and numerical methods have been successfully employed to determine the stoichiometries for the extraction of La3+ with mixtures of HEHEHP and CA100. The mixtures have different extraction effects on different rare earths, which provides the possibility for the separation of yttrium from heavy rare earths at an appropriate ratio of HEHEHP and CA100. (C) 2009 Society of Chemical Industry