The thermal decomposition of SF5O3SF5 in the presence of CO has been investigated between ‐9.8°C and + 9.9°C. Besides traces of S2F10, equimolecular amounts of SF5O2SF5 and CO2 are formed. (Formula Presented.) The reaction is homogeneous. Its rate is proportional to the pressure of the trioxide and in dependent of the total pressure, the pressure of inert gases and of carbon monoxide: (Formula Presented.) where k = k1∞ = 1016.32±0.40 exp(−25,300 ± 500 cal)/RT sec−1. Consequently, (Formula Presented.) In the presence of oxygen a sensitized CO2 formation is observed. A mechanism is given which explains the experimental results. Copyright © 1979 John Wiley & Sons, Inc.