A comparative study of ionic liquids (ILs) and a conventional organic solvent on the extraction of rare-earth ions from a nitrate-contained solution with TOPO has been investigated. A slope analysis was carried out to determine the extraction mechanism, and it was found that more TOPO molecules and the ionic constituents of the ILs have taken part in the extraction reaction in the ILs-based TOPO extraction systems. The numbers of TOPO molecules coordinated to the metal ion in the extracted complexes were 6 and 4 in the [C₄mim] and [C₈mim][Tf₂N] systems, although 3 TOPO molecules were needed in the n-dodecane system. Higher extraction efficiency was obtained in the extraction system in which the higher coordinating number of TOPO was shown. The extraction ability of [C₈mim][Tf₂N]-based TOPO depends significantly on the ionic constituents of the ILs in the extracting phase, but depends only slightly on the anionic nitrate ion from the aqueous phase, and [C₄mim][Tf₂N]-based TOPO is not dependent on the nitrate ion.