Abstract: The solvent extraction behaviour of the 15 rare earth elements (III)in the form of ternary complexes with varities of the bidentate heterocyclic amine ligands L (where L = 4,7-diphenyl-1,10-phenanthroline, 5-NO-1, 10-phenanthreline, 2,9-dimethyl-l, 10-phenanthroline; 2,9-dimethyl-4, 7-diphenyl-l, 10-phenanthroline and 2,2-bichinoline, respectively) and anion A (where A = trichloreacetate anion, CCl3COO-) as extractants has been investigated in the system of chloroform-water. In all the systems, the 'tetrad effect' hasbeen observed apparently during the extractions. As pHvalues in the media vary, the extraction effectiveness of the RE (III) will change. The chemical composition of the extracted compounds has been determined as RE(CCl₃COO)₂ A·2DPP (where A=Cl^- or CH₃COO^-, DPP = 4,7-diphenyl-l, 10-phenanthroline in the system of RE³⁺ /HCl-H₂O/ CCl₃COO^--DPP/CHCl₃, buffering by CH₃COOH-CH₃COONa.) The effects of molecular weights and structures of Lon extraction have been found as follows:The extraction effectiveness of RE (III) is raised with the increasing of molecular weights of ligands L, and reduced with the introduction of the -NO₂ group attracting electrons into L's molecules. The steric hindrance of the ligands Lwill greatly inhibit the extraction of the RE (III). The determination of the distribution ratios Dof RE (III) between chloroform and water phases is carried out by means of spectrophotometric method, the reagent arsenazo-Ibeing used as an indicator. The results have been tested by radiometric analysis, the isotopes¹⁴⁷Ce,^152,154Eu and ¹⁷⁰Tm being empl, yed, if necessary.