Synergistic solvent extraction of selected trivalent lanthanoids (La, Nd, Eu, Ho and Lu) with mixtures of a chelating extractants (HL), either 4-(4-fluorobenzoyl)-3-methyl-1-phenyl-pyrazol-5-one (HPMFBP) or 3-methyl-4-(4-methylbenzoyl)-1-phenyl-pyrazol-5-one (HPMMBP) and 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(dimethylphosphinoylmethoxy)calix[4]arene (S) in CHCl3 from chloride medium has been studied. The interaction between the extractants in deuterochloroform has been examined by H-1, C-13 and P-31 NMR spectra. The composition of the extracted species has been established as LnL(3)center dot HL (HL and L- represent either HPMFBP or HPMMBP and their anions, respectively). It has found that, in the presence of the phosphorus-containing calix[4]arene, the lanthanoids were extracted as LnL(3)center dot S. On the basis of the experimental data, the values of the equilibrium constants have been calculated. The influence of the para-substituted 4-aroyl-3-methyl-1-phenyl-pyrazol-5-ones on the extraction process has been discussed. The synergistic enhancement and separation factors between metals have been evaluated. (C) 2012 Elsevier B.V. All rights reserved.