Chemical-looping combustion is a promising technology that concentrates CO2 and separates it during combustion. In this study, both the carbon deposition and sulfur evolution in the reduction of a calcium sulfate (CaSO4) oxygen carrier with a typical syngas were investigated using thermodynamic simulations. The effects of reaction temperature, operating pressure and the oxygen ratio number (defined in this paper) on the amount of deposited carbon and released sulfurous gases are discussed. A reaction temperature from 750 to 950 degrees C, an operating pressure from 1 to 15 bars and an oxygen ratio number between 0.4 and 0.8 were determined to be the most favorable operating conditions. In addition, the amounts of released sulfurous gases were found to be largely dependent on the partial pressures of H-2 and CO based on the thermo-gravimetric analyzer (TGA) tests. When the partial pressure of H-2 or CO was above 40 kPa, the release of sulfurous gases could be prevented in the reaction between CaSO4 and syngas, even if the reaction temperature was as high as 1000 degrees C. The XRD profiles of the products also demonstrated that the mole fraction of CaS in the products increased gradually with an increasing partial pressure of H2 or CO, until the products were almost pure CaS. Crown Copyright (C) 2010 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers. All rights reserved.