This study demonstrates for the first time the possibility of selective separation of heavy, middle and light lanthanide ions, represented by Yb(III), Gd(III) and La(III), using a polymer inclusion membrane (PIM) composed of 45 wt% di-(2-ethylhexyl) phosphoric acid (D2EHPA) and 55 wt% poly(vinyl chloride) (PVC). Complete and selective extraction of these three lanthanide ions was achieved by varying the pH of the sulfuric acid feed solution (i.e. Yb(III) at pH 0.25, Gd(III) at pH 1.25 and La(III) at pH 2.25). The extracted ions were back-extracted completely into solutions containing sulfuric acid at concentrations of 7.0, 1.0 and 0.3 mol L-1, respectively. A comprehensive study of the Yb(III) extraction system revealed that the Yb(III): (D2EHPA) 2 ratio in the extracted complex was 1: 3, where (D2EHPA) 2 refers to the D2EHPA dimer. The corresponding thermodynamic extraction constant was determined to be 7.45 x 10(4). Assuming the same stoichiometry for the La(III) and Gd(III) complexes, the thermodynamic extraction constants of these two lanthanides were determined on the basis of their extraction isotherms as equal to 0.776 and 81.2, respectively. The initial flux values for the three lanthanide ions studied ranged from 6.98 to 3.85 x 10(7) mol m(-2) s(-1) in the extraction experiments and from 12.4 to 5.76 x 10(7) mol m(-2) s(-1) in the back-extraction experiments.