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 βvs 100/T indicates that the enthalpy does not change much and is calcula...