Cesium fluoride reacts with SO2 under an ultrasonic treatment, with the formation of Cs2S3O6 and SO2F2 instead of CsSO2F. The trithionate salt crystallizes with one solvent molecule (Cs2S3O6.SO2) in the monoclinic space group P2(1)/c with a = 9.702(3) Angstrom, b = 15.315(6) Angstrom, c = 7.558(4) Angstrom, beta = 97.17(4)degrees and 4 formula units per unit cell. The reaction route for the CsF/SO2 system is discussed with regard to the activation or reactivation of CsF as catalyst.