The liquidliquid equilibrium (LLE) of a ternary system of water (H2O) + 2-propanol + 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Dmim][NTf2]) ionic liquid was measured in the temperature range from 278.15 to 298.15 K at atmospheric pressure. The distribution coefficient and selectivity were calculated to determine the influence of the temperature on the LLE. The results indicate that a decrease in temperature reduces the solubility of H2O in [Dmim][NTf2] and increases the immiscibility region of the ternary phase diagram. The experimental data were correlated using the nonrandom two liquid and the universal quasichemical models, and the critical properties for [Dmim][NTf2] were determined.