The IR (gas) and Raman (liquid) spectra of FC(O)NSCl2 demonstrate the presence of a conformational mixture in both phases. According to a gas electron diffraction study, the main conformer (94(8)%) possesses a syn-syn structure (C(O)F group synperiplanar with respect to the SCl2 bisector and the C=O bond synperiplanar to the N=S bond). Quantum chemical calculations (HF, B3LYP and MP2 with 6-31G* basis set, and MP2/6-311(2df)) predict a syn-anti structure for the second conformer. Analysis of the IR (gas) spectrum results in a contribution of 5(1)% of the minor form, corresponding to a Gibbs free energy difference DeltaGdegrees = Gdegrees(syn-anti) - Gdegrees(syn-syn) = 1.75(15) kcal/mol. This value is reproduced very well by quantum chemical calculations, which include electron correlation effects (DeltaGdegrees = 1.28-1.56 kcal/mol). The HF approximation overestimates this energy difference (DeltaGdegrees = 3.24 kcal/mol).