In aqueous medium, selective complexation increases the ionic separation by nanofiltration. Within the context of the nuclear effluent treatment, the nanofiltration-complexation association is applied to sodium/cesium separation. Resorcinarene, a water-soluble ligand of cesium, is associated to the FILMTEC NF 70 membrane. Effects of pH, transmembrane pressure, ligand concentration, and ionic strength on salts retention and filtration fluxes are described. Finally, cesium complexation constants with resorcinarene are determined by using the nanofiltration-complexation process.