The geometric structures and conformational properties of (fluorocarbonyl)trifluoromethylsulfane, CF3SC(O)F, and (chlorocarbonyl)trifluoromethylsulfane, CF3SC(O)Cl, were determined by gas electron diffraction (GED), vibrational spectroscopy (IR(matrix)), and theoretical calculations (HF/3-21G*, HF/6-31G*, MP2/6-31G* ab initio, and BPW91/6-31G*, B3PW91/6-31G* density functional methods). For CF3SC(O)F a mixture of planar syn and anti conformers occurs with Delta G(o)(GED) = G(o)(anti) - G(o)(syn) = 1.3(4) kcal mol(-1) and Delta H-o(IR) = H-o(anti) - H-o(syn) = 1.43(12) kcal mol(-1). Only the planar syn form is obtained in the case of CF3SC(O)Cl. All theoretical approaches reproduce the geometric parameters and conformational properties of both compounds very well. The predicted energy differences Delta E = E(anti) - E(syn) vary from 0.98 to 1.24 kcal mol(-1) for CF3SC(O)F and from 2.96 to 3.69 kcal mol(-1) for CF3SC(O)Cl.