In order to increase the solvent extraction and separation of light rare earth elements (La, Ce, Pr and Nd) from chloride leach solution of monazite sand, the binary mixtures containing acidic (D2EHPA, PC88A and Cyanex 272) and tertiary amine (Alamine 336, TOA and TEHA) have been employed. Among the studied extractant mixtures, the combination of Cyanex 272 and Alamine 336 showed that the best extraction ability and the maximum synergistic enhancement factors. The addition of TBP into the binary mixtures also resulted in remarkable synergism on the extraction of REEs and enhanced the stripping of REEs. The role of Alamine 336 in this mixture was to extract hydrogen ions released from acidic extractant, while the addition of TBP to the binary mixture promoted the extraction of REEs by forming adducts with the extracted species. The overall solvent extraction reaction was identified by applying a slope analysis method. The nature of interaction between the extractants in the binary mixtures which affects synergism in extraction was investigated by comparing the extraction of acid by different binary mixtures. It was found that the interaction between strong acidic extractant (D2EHPA) and Alamine 336 was strong enough to form an amine salt which took part in the reaction. In the binary mixture of weak acidic extractant (Cyanex 272 and PC88A) and Alamine 336, each extractant took part in the reaction independently. IR spectroscopy of the organics confirmed the interaction and the strength of it in the binary mixtures. The extraction of LREEs increased with the increase of dielectric constant of diluents. The interaction of components in binary mixtures diluted in different diluents are studied by comparing their IR spectra, viscosity, and excess viscosity.