Abstract: 2-Ethylhexylphosphonic acid mono(2-ethylhexyl)ester(EHPAEH)is a more effective extractant than D2EHPA for extraction of rare earths.Fifteen solid and solution rare earth-EHPAEH complexes in kerosine have been prepared and their FT-IR spectra have also been studied usinga subtraction technique. In the FT-IR subtraction spectra of either the extracted species in the organic phase or the solid complexes, a positive peak and a negative onewere observed corresponding to ν_P=O→M and ν_P=O respectively, or to ν_P-O-Mand ν_P-O-H respectively. The variation of ν_P=O→M, ν_P-O-M and ν_O-M(150cm^-1)with the atomic number of lanthanides was shown in Fig.5 and Fig.7 for the solid complexes. These curves show an obvious "tetrad effect", Y being located between Er and Tm.