The extraction of rare earth elements (REEs) was investigated with mixtures of sec-octylphenoxyacetic acid (CA 12. H(2)A(2)) and 2,2'-bipyridyl (bipy, B) from nitrate medium. The mixing system had synergistic effects on all of the studied REEs and might he employed to the separation of REEs at proper extractant ratios. Methods of slope analysis and constant mole were used to determine the extracted compounds for La and Y as La(NO(3))(2)H(3)A(4)B and Y(NO(3))(2)H(2)A(3)B, respectively. Thermodynamic parameters Delta H, Delta G, and Delta S were also calculated, indicating that both the extraction processes of La and Y were endothermic reactions.