A new lower rim substituted calix[8]arene bearing eight phosphine oxide moieties has been synthesized. The structure of 5,11,17,23,29,35,41,47-octa(1,1,3,3-tetramethylbutyl)-49,50,51,52,53,54, 55,56-octakis(dimethylphosphinoylmethyleneoxy)calix[8]arene (S) was identified and confirmed by elemental analysis, IR-, H-1, C-13 and P-31-{H-1} NMR spectroscopy as well as by ES-mass spectrometry. The synergistic solvent extraction of five selected lanthanoid ions (La3+, Nd3+, Eu3+, Ho3+ and Lu3+) with 4-benzoyl-3-methyl-1-phenyl-5-pyrazolone (HP) and S in chloroform has been studied. It was found that in the presence of this phosphorus-containing calix[8]arene the lanthanoids have been extracted as LnP(3).S. On the basis of the experimental data, the values of the equilibrium constants have been calculated. The influence of the synergistic agent (S) on the extraction process has been discussed. A synergistic effect of about three orders of magnitude occurs in the extraction of Ln(III) with the mixture of HP and S. The values of the separation factors between the adjacent elements have been evaluated. (C) 2008 Elsevier B.V. All rights reserved.