Pure chlorocarbonyl trifluoromethanesulfonate, ClC(O)OSO2CF3, has been prepared in about 58% yield by the ambient-temperature reaction between ClC(O)SCl and AgCF3SO3. The conformational properties of the gaseous molecule have been studied by vibrational spectroscopy [IR(gas), IR(matrix), and Raman(liquid)] and quantum chemical calculations (HF and B3LYP with 6-31+G* basis sets); in addition, the solid-state structure has been determined by X-ray crystallography. ClC(O)OSO2CF3 exists in the gas phase as a mixture of trans [ClC(O) group trans with respect to the CF3 group] and gauche conformers, with the trans form being the more abundant [66(8)% from IR(matrix) measurements]. In both conformers, the C=O bond of the ClC(O) group is oriented synperiplanar with respect to the S-O single bond. The experimental free energy difference between the two forms, DeltaGdegrees = 0.8(2) kcal mol(-1) (IR), is slightly smaller than the calculated value (1.0-1.5 kcal mol(-1)). The crystalline solid at 150 K [monoclinic, P2(1)/n, a = 7.3951(9) Angstrom, b = 24.897(3) Angstrom, c = 7.4812(9) Angstrom, beta = 99.448(2)degrees, Z = 8] consists surprisingly of both trans and gauche forms. Whereas the more stable conformer for the more or less discrete molecules and the polarization effects would tend to favor the trans form, the packing effects would stabilize the gauche rotamer in the solid state.