The liquid-liquid extraction of some trivalent rare earths Ln(III) (Ln: La, Eu, Lu), performed from 0.1 M NaNO3 aqueous solutions by two bis(4-acyl-5-hydroxypyrazoles), 1,12-bis(1'-phenyl-3'-methyl-5'-hydroxy-4'-pyrazolyl)-dodecane-1,12-dion e (HL-10-LH) and 1,6-bis(1'-phenyl-3'-methyl-5'-hydroxy-4'-pyrazolyl)-hexane-1,6-dione (HL-4-LH), in chloroform, has been studied. The processes of extraction have been determined by the slope analysis method and by analyzing a function that allows the simultaneous treatment of all the experimental points obtained in different conditions. With HL-10-LH, the stoichiometries of the extracted species differ for the three lanthanides. The ligands are biscoordinated and tetracoordinated in the extracted complexes of lanthanum and europium, La-2 (L-10-L)(2) (L-10-LH)(2) and Eu-n (L-10-L), (L-10-LH)(n) (n = 1,2). They are exclusively biscoordinated in the complex of lutetium, Lu(L-10-LH)(3). The complex of europium extracted by HL-4-LH at weak extraction yields, Eu(L-4-L)(L-4-LH), and the corresponding extraction constant are comparable to the ones obtained for HL-10-LH, whereas polynuclear species are extracted at high extraction yields. The extraction efficiency with HL-10-LH increases with the ascending atomic number of Ln (La < Eu < Lu). A comparison between HL-10-LH and HPMBP (1-phenyl-3-methyl-4-benzoyl-5-hydroxy-pyrazole) for the same concentration of complexation sites shows that the extraction by HL-10-LH is more effective. Moreover, a better selectivity of extraction was obtained with HL-10-LH. (C) 2005 Elsevier B.V. All rights reserved.