Abstract: The complexation reaction of Ln(Ⅲ)/TBP (Ln=Nd, Eu) in water-saturated 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide (C₄mimTf₂N) was investigated by absorption spectrophotometry and fluorescence spectroscopy. The results indicate that the stability constants (lg β) of the 1∶1 complex of Nd(Ⅲ) with tributyl phosphate (TBP) are 0.68±0.03, 0.76±0.02, 0.89±0.03, 0.97±0.03, 1.04±0.03 at 10, 25, 40, 55 and 70 ℃ respectively. The best linearity of the plot of ln β vs 100/T indicates that the enthalpy does not change much and is calculated to be 11.52 kJ/mol in the range of 10-70 ℃. The stability constant of the 1∶1 complex of Eu(TBP)³⁺ is 0.78±0.06, which is a little higher than that of the Nd(TBP)³⁺ complex and can be ascribed to the lanthanides contraction.