In the present study, the extraction of rare earths with mixtures of 1-phenyl-3-methyl-4-benzoylpyrazalone-5 (HPMBP, HA) and sec-octylphenoxyacetic acid (CA12, H2B2) in benzene has been studied from chloride medium. The synergistic enhancement coefficient decreases with increasing atomic numbers of rare earths. The synergistic extraction of lanthanum, and neodymium has been studied with the methods of slope analysis and constant mole. The extracted complexes are determined as LaH(2)ClA(2)B(2) and NdH(3)ClA(3)B(2), respectively. The equilibrium constants, formation constants, and the thermodynamic functions are calculated. Furthermore, the different extraction effects on rare earths have been employed to discuss the possibilities for the separation of rare earths. (c) 2009 Elsevier B.V. All rights reserved.