In the present work, new data on the densities and refractive indices for 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate hmimFAP with N-methyldiethanolamine are reported for various concentrations and at temperatures (303.15-328.15) K. Excess molar volumes {V^{\text{E}}} V E and excess refractive indices n-{\text{D}}^{\text{E}} n D E were calculated from the experimental data. Refractive index values for the binary mixtures were predicted by using Lorentz-Lorenz, Gladstone-Dale and Eykman equations. Excess molar volumes showed positive trend, whereas excess refractive indices showed negative trend over the entire range of temperatures and concentrations studied in the present research. © 2015 Akadémiai Kiadó, Budapest, Hungary.