Sulfurtrioxide reacts with the superacidic solutions XF/SbF(5) (X=H, D) to form the corresponding salts [X(2)SO(3)F](+)[SbF(6)](-), which are the protonated forms of fluorosulfuric acid. The salts have been characterized by vibrational spectroscopy and a single-crystal structure analysis. [H(2)SO(3)F](+)[SbF(6)](-) crystallizes in the monoclinic space group P2(1)/n (no. 14) with four formula units in the unit cell. The crystal structure possesses a distorted tetrahedral O(3)SF skeleton of the cations, which are linked with two strong hydrogen bridges to [SbF(6)](-) anions and forms a one-dimensional chain. The crystal structure and the vibrational spectra are compared to the quantum-chemical-calculated free [H(2)SO(3)F](+) cation. Additionally, an [H(2)SO(3)F(HF)(2)](+) unit was calculated at the RHF/6-311(++)G(d,p) level to simulate HF hydrogen bridges found in the solid state.