In gasification processes it is possible to capture H2S by limestone (CaCO3) or dolomite (CaCO3 MgCO3). In either case the desulfurization reactions produce the unstable product CaS. CaS has to be stabilized before it can be disposed of since it may react with water. One way to stabilize the CaS is to oxidize it to CaSO4. In this work the stabilization of CaS by oxidation has been studied with a pressurized thermobalance. Experiments were done to study the effects of p(tot), p(O2), temperature, and sulfidation degree. Some of the solid samples were further analyzed for total sulfur. It was found that an increase in total pressure also increased the conversion to CaSO4. This could not simply be explained with the change in p(O2), but it was speculated that small amounts of released SO2 increased the stability of CaSO4 with increasing pressure. Higher O-2 concentration was also found to accelerate the conversion rate, but the effect was limited and even with an increase from 5 to 21 vol %, no significant difference could be observed. This was attributed to the fast production of a dense CaSO4 layer, hindering any further reaction. It was also found that a decrease in sulfidation degree resulted in a higher conversion rate to CaSO4 and, as earlier found at atmospheric pressure, also an optimal temperature below 950 degrees C at 20 bar. Overall, it was concluded that CaS produced from dolomite has a significantly higher tendency to oxidize to CaSO4 than CaS from limestone. The maximum stabilization degrees were about 80% and 50%, respectively.