Ab initio calculations were combined with infrared and Raman studies to distinguish spectroscopically the two conformers of the bis(trifluoromethanesulfonyl)imide anion, (TFSI^-). Spectra of crystalline LiTFSI complexes with organic ligands, where the anion adopts a known conformational state, are presented to confirm the calculated spectra. Several regions are identified where either the infrared or the Raman spectra contain separate bands for the two conformers. The conformational equilibrium between the transoid and cisoid rotamers is then illustrated from the infrared spectra of solutions of LiTFSI in aprotic solvents. The transoid form is found to be more stable than the cisoid form by about 2.2 kJ mol^-1, in good agreement with the present and earlier theoretical predictions. It is also shown that the IR and Raman spectral changes coming from conformational isomerism have to be carefully distinguished from those due to ionic interactions.