Abstract Desulfurization at medium temperature was investigated with focus on impacts of temperature and CO 2 concentration under Oxyfuel firing and Regenerative Calcium Cycle (RCC) conditions. Investigation started with thermodynamic analysis and ThermoGravimetric Analysis (TGA). Tests were then conducted by a bench-scale fixed-bed type reactor, using a synthetic gas mixture. Various parameters were evaluated. The envelope covered temperature variation of 150-450 °C, CO 2 concentrations in the gas stream up to 60% and water concentrations between 0 to 25%. These tests were done having SO 2 concentrations in the gas of 2000 ppm SO 2 . Furthermore different types of lime samples were screened. It was found that CO 2 had strong impact on medium temperature desulfurization. Capture of CO 2 by the lime was enhanced by increasing temperature. CO 2 competed with SO 2 and the relative extent to react with lime varied with temperature. Rate of reaction increased with temperature, the capacity of lime-to-SO 2 and lime-to-CO 2 increased as well. Optimum temperature windows were searched and will be discussed. Based on the tests results, scenarios of desulfurization options at medium temperature range for Oxy and RCC are evaluated, and will be discussed in details. The testing results show issues related to high concentration of CO 2 under the Oxy conditions at temperature over 250 °C. Though capacity of lime for SO 2 increases with temperature, the amount is limited due to reaction with CO 2 at high concentration. Desulfurization at medium temperature range, 250 - 450 °C, should be aiming at moderate SO 2 removal, and the reuse of the spent reagent is considered in the downstream wet desulfurization process. With about 10% CO 2 , in the RCC conditions, it is more optimistic to remove SO 2 in the medium temperature range. Optimum temperature range is discussed for achieving moderate SO 2 removal. Reuse of the spent reagent is considered in the downstream wet desulfurization process.