The solvent extraction of lanthanum(III) and lutetium(III) from 0.1 M nitrate medium with monoacidic chelating extractants of various acidities and lipophilicities, in chloroform, at 25 degrees C, has been studied and compared with previously reported extraction data for europium. The extractants, "HL", are either the 3-phenyl-4-p-tert-butylbenzoyl-isoxazol-5-one, "HPtbBI", or the substituted 4-acyl-5-hydroxypyrazoles 1-(p-nitro-phenyl)-3-phenyl-4-lauroyl-5-hydroxypyrazole, "HNPPLP", and 1-phenyl-3-methyl-4-(p-tert-butylbenzoyl)-5-hydroxy-pyrazole, "HPMtbBP". Like europium, lanthanum and lutetium are extracted by cation exchange as LnL(3.org). Because of the higher acidity of the former ones, extractions performed with 4-acylisoxazol-5-ones occur at lower pH values than with 4-acyl-5-hydroxypyrazoles. The order of extraction efficiencies is: HPMtbBP (pK(a) = 4.2)< HNPPLP (pK(a) = 3.2)< HPtbBI (pK(a) = 1.3)< HPBI (pK(a) = 1.2) (HPBI: 3-phenyl-4-benzoyl-isoxazol-5-one). Moreover, for all these chelators, the extraction efficiency follows the order lanthanum < europium < lutetium, while their separation factors decrease with increasing extractant acidity. The extractions of other trivalent lanthanoids (Ln(3+) = Ce3+, Nd3+, Sm3+, Tb3+, Ho3+ and Tm3+) and yttrium(III) from nitrate medium with HNPPLP have also been investigated. The equilibrium constants and the intra-lanthanoidic separation factors show the same trends as those observed for other beta-ketoenol extractants, HPMBR 1-phenyl-3-methyl-4-benzoyl-5-hydroxypyrazole, and HTTA, thenoyltrifluoroacetone. The overall selectivity, SF(Lu/La), is discussed with respect to the extraction thermodynamics, the d(O center dot center dot center dot O)(HL) structural parameters, measured or calculated by semi-empirical methods, and the inner- or outer-sphere nature of the extracted complexes. (c) 2008 Elsevier B.V. All rights reserved.