Pure fluorocarbonyl trifluoromethanesulfonate, FC(O)OSO2CF3, is prepared in about 70% yield by the ambient-temperature reaction between FC(O)SCI and AgCF3SO3. The geometric structure and conformational properties of the gaseous molecule have been studied by gas electron diffraction (GED), vibrational spectroscopy [IR(gas), IR(matrix), and Raman(liquid)] and quantum chemical calculations (HF, MP2, and B3LYP with 6-311G* basis sets); in addition, the solid-state structure has been determined by X-ray crystallography. FC(O)OSO2CF3 exists in the gas phase as a mixture of trans [FC(O) group trans with respect to the CF3 group] and gauche conformers with the trans form prevailing [67(8)% from GED and 59(5)% from IR(matrix) measurements]. In both conformers the C=O bond of the FC(O) group is oriented synperiplanar with respect to the S-O single bond. The experimental free energy difference between the two forms, DeltaGdegrees = 0.49(13) kcal mol(-1) (GED) and 0.22(12) kcal mol(-1) (IR), is slightly smaller than the calculated value (0.74-0.94 kcal mol(-1)). The crystalline solid at 150 K [monoclinic, P2(1)/c, a = 10.983(1) Angstrom, b = 6.4613(6) Angstrom, c = 8.8508(8) Angstrom, 104,786(2)degrees] consists exclusively of the trans conformer.