Calcium sulfide is an undesirable product from both FBC and topping cycle gasifiers. Unfortunately, its direct oxidation in a FBC environment is made difficult due to the fact that unreacted CaS is protected from further reaction with O-2 by formation of a CaSO4 shell. In previous work it was often implied that oxidation of CaS occurred solely by reaction with O-2. However, this study shows that CO2 can be an effective oxidant for CaS at temperatures above 600 degreesC. Furthermore, it appears that the principal gas-phase products are SO2 and CO, and direct attempts to measure COS using mass spectrometry suggest that its formation is negligible. Somewhat surprisingly, SO2/CO molar ratios are in the range of 0.4 to 0.5, which is problematic, if the dominant reaction is CaS + 3CO(2) = CaO + 3CO + SO2, unless CO2 is reacting to form elemental carbon, which appears unlikely, or side reactions occur, leading to a variety of gaseous products. These results also suggest that CO itself has no significant part in the oxidation process at least up to 850 degreesC or more. Reaction with H2O also begins above 600 degreesC, but it is a much less effective oxidant, and experiments with CO2/H2O further support the idea that CO2 is the more important oxidant and might be used to destroy CaS from topping cycle gasifiers.