[en] Lanthanides(III)/actinides(III) separation is a major research subject in matter of treatment of high activity liquid effluents. Liquid-liquid extraction actually gives the best results for this separation. In order to demonstrate that nano-filtration (NF) is a valuable alternative to liquid-liquid extraction, we tried to separate different lanthanides(III) with a nano-filtration process combined with a selective complexation step. At first DTPA (diethylene-triamine-pentaacetic acid) combined with a Sepa MG-17 (Osmonics) gave a 95% retention of Gd³⁺ and a 50% retention of La³⁺. Then new hydrosoluble and more selective ligands derived from DTPA were synthesized. One of them combined with a Sepa MG-17 membrane allowed a 87% retention of Gd³⁺ and a 5% retention of La³⁺. The same nano-filtration-complexation system was experimented with an equimolar aqueous solution of Gd³⁺, Pr³⁺ and La³⁺. Other experiments in the field of actinides(III)/lanthanides(III) separation were also performed. (authors)