The structure of SF5OSO2F has been investigated using gas-phase electron diffraction and quantum-chemical calculations. It is found to exist primarily in the gauche form (SF5 group gauche relative to the S-F bond of the SO2F group with phi(S-O-S-F = 71(7)degrees). A small contribution of the trans conformer cannot be excluded. Photoelectron spectroscopy gives ionization energies for the sulfur 2p electrons that reflect the relative electronegativities of fluorine and oxygen. The widths of the peaks in the photoelectron spectra indicate that there is considerable vibrational excitation associated with the core ionization of the sulfur atoms.