The solvent extraction of trivalent lanthanoids (without Pm) with thenoyltrifluoroacetone (HTTA) and dibenzo-18-crown-6 (DB18C6) or dibenzo-24-crown-8 (DB24C8) has been studied. It was found that in the presence of a crown ether (S) the lanthanoids have been extracted as Ln(TTA)(3) (.) S. On the basis of the experimental data, the values of the equilibrium constants have been calculated. It was established that the stability of the complexes involving DB24C8 is higher than those involving DB18C6. The influence of the synergistic agent on the extraction process has been discussed. The synergistic and separation factors have been determined too.