The use of nuclear magnetic resonance (NMR) spectroscopy to elucidate the structure of complexes formed in liquid-liquid extraction is reported. 3IP spectral information provided substantive evidence for the structural difference of the isolated solid complex and the complex in solution. Solvent effect exists for the31P chemical shift of the lanthanum-2-ethylhexyl phenylphosphonate in n-hexane, toluene and tetrahydrofuran, as well as in the extractability of lanthanum from aqueous perchloric acid solution.