Abstract: The extraction behaviour of rare earth elements(Ⅲ) from nitric acid soltition by HEH[EHP]-hexane has been systematically investigated. The mechanism of extraction carried out in different acidities is discussed in terms of slope, IR and NMR data obtained. It is concluded that at low aqueous acidity, rare earths(Ⅲ) are extracted by a cation-exchange reaction with hydrogen liberation: (1)and at high aqueous acidity the extraction is accompanied by a solvation reaction similar to that for TBP; (2) The influences of temperature and solvent upon the extraction equilibrium under different conditions were discussed in detail. The equilibrium constants (K₁ anb K₂) and the thermodynamic functions (ΔH, ΔZ_z and ΔS_z)of extraction reaction were calculated. The dependency of K,ΔZ_z and ΔS_z on the atomic number of rare earth elements were also discussed.