Chemical-looping Combustion (CLC) has been proposed as an energy-efficient combustion method for in situ capture of CO2. Kinetic model for parallel reactions of the CaSO4 oxygen carrier with CO in a CLC process is explored in this paper. Tests on an isothermal reaction were carried out in a Thermogravimetric Analyzer coupled with Fourier Transform Infrared spectrum (TGA-FTIR), and the instantaneous evolutions of SO2, and COS were monitored by the FTIR quantitative analysis. The reaction temperature was varied between 850 and 1050 degrees C, while 5-28% CO concentrations were utilized. The experiments showed that the reduction of CaSO4 by CO was a complex process, with the products of either sole CaS or both CaS and CaO depending on the reaction temperature as well as the concentration of CO reactant. The parallel reactions of CaSO4 with CO were investigated in terms of the selectivity based on the nucleation and growth model under isothermal conditions. According to the fitting results, the nucleation and growth model fit well the conversion-time data, and some of the kinetic parameters were obtained.