Photochemically induced gas-phase reactions of Cl-2/SO2/O-2 mixtures at -78 degrees C have been investigated on a preparative scale. The sulfuryl chlorides ClSO2(OSO2)(n)Cl, n = 0, 1, or 2, along with SO3 and the hitherto unknown bis(chlorosulfuryl)peroxide, ClSO2OOSO2Cl, have been obtained. The new peroxide has been purified by trap-to-trap distillation and conclusively characterized by gas-phase and matrix IR, as well as Raman, spectroscopy. At room temperature, the peroxide decomposes to form quantitatively Cl-2 and SO3. DFT calculations suggest that ClSO2OOSO2Cl occurs at ambient temperatures in at least two different rotamers, whereas in matrix-isolation experiments in Ar at 15 K only the most stable form has been found. The mechanism of the formation of the peroxide via the ClSO2OO center dot peroxy radical and the supposed involvement of the ClSO3 center dot intermediate is discussed.