The effect of adding a water-soluble chelating agent, diethylenetriaminepentaacetic acid (DTPA), to the feed phase was studied on the selective permeation of rare earth metal ions (Ln3+) through a supported liquid membrane (SLM) containing 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester (PC-88A). High selectivity of Pr to Nd was observed at a pH of 2.0 in the stripping phase, and selective permeation of La was observed at higher pH values. The permeation mechanism of the Ln3+ was analyzed, and the above results were explained by the change in the concentrations of free Ln3+ in the feed phase and Ln3+/PC-88A complexes in the SLM with pH.