A B S T R A C T The infrared absorption spectrum of the N,N-diethyl-N-methyl-N-propylammonium (DMPA) bis (fluorosulfonyl)imide (FSI) ionic liquid is measured as a function of temperature between 165 and 307 K. In the frequency range between 900 and 1070 cm -1 only the cation gives rise to infrared bands. A conformational analysis of DMPA is performed by means of DFT calculations at the B3LYP/6-31G** level. The comparison of the experimental and calculated spectra provides evidence that the lowest energy conformers coexist in the liquid. Experimentally, we find that the energy difference between the most stable rotamer and the next energy conformers is only 1.56 +/- 0.05 kJ/mol, in good agreement with the calculated value. Finally, we show that in the solid state only the most stable isomer is retained.