Distribution ratios of trivalent lanthanide ions in extraction reactions may be affected by the denticity of a solvent extraction ligand for entropic reasons. This hypothesis was tested by studying the reactions of extraction of the lanthanides(III) (La, Nd, Eu, Dy, Er and Yb) from perchlorate media into methyl isobutyl ketone (MIBK) with three structurally related malonamide ligands (I - III): 2,2 '-[1,2-phenylenebis(methylene)]bis(N,N,N',N'-tetrabutylmalonamide) (I), 2,2 '-[1,4-phenylenebis(methylene)]bis(N,N,N',N'-tetrabutylmalonamide) (II), and 2-benzyl-N,N,N',N'-tetrabutylmalonamide (III), the molecules of which contain either two (III) or four (I and II) carbonyl oxygen donor atoms. Ligands I and II are the structural isomers. The extracted species in the reactions with all these ligands have the (1:2) metal-to-ligand stoichiometric ratios. All the extraction reactions are enthalpy-driven and have negative values of the standard molar entropy changes at T = (298-313) K. Since the least exothermic standard molar enthalpy changes of the extraction reactions (Delta H-r degrees(T) ) and the least negative standard molar entropy changes of the reactions (Delta S-r degrees(T)) were observed for ligand I, it was concluded, that the distribution ratios of the lanthanide(III) ions were the highest in the reactions with I due to the entropic effect, presumably resulting from the difference in the coordination of the extraction ligands with the lanthanide(III) ions. (C) 2018 Elsevier Ltd.