Infrared spectra in the gaseous state, and in the matrix and Raman spectra in the solid state, were recorded for the halogen fluorosulfates XOSO 2F, X = F, Cl. The absence of frequency splittings in the matrix spectra demonstrates the presence of one single conformer for both compounds. The assignments of the fundamental vibrations are supported by normal coordinate analyses based on frequencies and isotopic shifts. The molecular structures of both compounds were determined by gas electron diffraction (GED) and ab initio calculations ( HF 3-21 G∗ and HF 6-31 G∗ . According to the GED analysis and in agreement with the ab initio calculations gauche conformers with dihedral angles τ (FSOX) ≈ 70° are preferred in both fluorosulfates. The calculations predict the trans forms, τ(FSOX) ≈ 180°, to be higher in energy by about 10 kJ mol −1 for X = F and by about 8 kJ mol −1 for X = Cl.