The synergistic solvent extraction of samarium (III) with mixtures of sec-octylphenoxy acetic acid (CA12, H(2)A(2)) and bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex301, H2B2) in n-heptane has been investigated from chloride medium. The methods of slope analysis and constant mole are used for the determination of the reaction. Samarium (III) is extracted with the mixture as SmH(2)Cl(2)AB(2) instead of SmA(3)center dot HA or SmB3 center dot HB when extracted with CA12 or Cyanex301 alone, respectively. The equilibrium constants and the thermodynamic functions, Delta H, Delta G, and Delta S have been calculated, indicating that the reaction is endothermically driven. Furthermore, the extraction effects of other rare earth elements are investigated to study the possibility of separating rare earth ions according to their separation factors. (C) 2008 Elsevier B.V. All rights reserved.