The solvent extraction of 14 lanthanoids with a thenoyltrifluoroacetone (HTTA) in the presence of tert-butylcalix[4]arene tetrakis(N,N-dimethylacetamide) (S) from perchlorate medium at constant ionic strength mu = 0.1 was investigated. The complexation properties of the extractants in the organic phase were examined and the parameters of the extraction process were determined. The stoichiometry of the extracted species was determined by a classical log-log plot analysis. It was found that the composition of the extracted species is [Ln(TTA)(3)S-2]. The presence of a calix[4]arene has caused a significant synergistic effect. The values of the synergistic coefficients as well as the values of separation factors of the ions of the entire 4f-series were calculated. The values of the equilibrium constants were calculated, too. The formation of a solid ternary complex of Eu(III), HTTA and S was studied using elemental analysis, IR, H-1 NMR. The stoichiometry of the complex was found to be Eu(TTA)(3).S. (C) 2011 Elsevier B.V. All rights reserved.