In an investigation of in situ CO2 removal for fluidized-bed combustion processes, seven calcium-based sorbents were tested for simultaneous CO2 and SO2 removal using both an atmospheric thermogravimetric reactor and a pressurized thermogravimetric analyzer. SO2 was found to impede cyclic CO2 capture because of pore blockage by sulfate products, resulting primarily from direct sulfation during the later stage of each cycle. The sorbents showed similar patterns during cocapture. Loss in sorbent reversibility could not be prevented but was improved by higher CO2 partial pressures.