The geometric structure and conformational properties of S-( fluoroformyl) O-( trifluoroacetyl) thioperoxide, FC( O) S- OC( O) CF3, were investigated by gas electron diffraction, matrix isolation infrared spectroscopy, and quantum chemical calculations ( B3LYP with the 6-31G* and aug-cc-pVTZ basis sets and MP2 with the 6-31G* basis set). The experimental methods result in a mixture of two conformers with gauche conformation around the S- O bond. In the main conformer ( 82( 7)% according to GED at 298 K), the C=O bond of the FC( O) group is oriented syn with respect to the S- O bond and phi( C-S-O-C)) 75( 3). In the minor conformer ( 18( 7)%), this C=O is oriented anti. Both conformers possess syn orientation of the C=O bond of the CF3C( O) group. The conformational properties and geometric parameters are reproduced reasonably well by the quantum chemical calculations, except for the S- O bond length, which is predicted too long by 0.04 angstrom ( B3LYP/aug-cc-pVTZ).