Anion conformation of a low-viscosity room-temperature ionic liquid 1-ethyl-3-methylimidazolium his(fluorosulfonyl) imide (EMI+FSI-) has been studied by Raman spectra and theoretical DFT calculations. Three strong Raman bands were found at 293, 328, and 360 cm(-1), which are ascribed to the FSI- ion. These Raman bands show significant temperature dependence, implying that two FSI- conformers coexist in equilibrium. This is supported by theoretical calculations that the FSI- ion is present as either C-2 (trans) or C-1 (cis) conformer; the former gives the global minimum, and the latter has a higher SCF energy of about 4 W mol(-1). Full geometry optimizations followed by normal frequency analyses show that the observed bands at 293, 328, and 360 cm(-1) are ascribed to the Q, conformer. The corresponding vibrations at 305, 320, and 353 cm(-1) were extracted according to deconvolution of the observed Raman bands in the range 280-400 cm(-1) and are ascribed to the C-2 conformer. The enthalpy Delta H degrees of conformational change from C-2 to C-1 was experimentally evaluated to be ca. 4.5 W mol(-1), which is in good agreement with the predicted value by theoretical calculations. The bis(ti-ifluoromethanesulfonyl) imide anion (TFSI-) shows a conformational equilibrium between C-1 and C-2 analogues (Delta H degrees = 3.5 kJ mol(-1)). However, the profile of the potential energy surface of the conformational change for FSI-(the F-S-N-S dihedral angle) is significantly different from that for TFSI- (the C-S-N-S dihedral angle).