The extraction of rare earths with mixtures of sec-nonylphenoxy acetic acid (CA100. H(2)A(2)) and 1,10-phenanthroline (phen, B) in benzene has been investigated from nitrate medium. The mixtures show synergistic effects on all the selected rare earths, lanthanum, neodymium, samarium, terbium, ytterbium, and yttrium. The extraction stoichiometries of samarium and yttrium with CA100 and its mixtures with phen have been studied in detail with the methods of slope analysis and constant mole. The equilibrium constants and thermodynamic functions are calculated. In addition, the mixtures are proved to have higher selectivity when applied to the separation of the lanthanoids from yttrium compared with single CA100 system. (C) 2009 Elsevier B.V. All rights reserved.