CaS, generated from H 2 S removal process using calcium-based sorbents during coal gasification, is a hazardous chemical. The unstable CaS can be converted to stable CaSO 4 via oxidation. Using Fourier Transform Infrared Spectroscopy (FTIR) technique, the experiments of CaS oxidation were performed in a thermogravimetric analyzer (TGA) qualitatively and quantitatively. FTIR coupled with TGA, which can provide in-situ data in real time, was applied to illuminate the SO 2 evolution in the product gases during CaS oxidation. There is a maximum concentration of SO 2 about 4 min after the CaS reaction starts. The FTIR analyses for the solid products obtained at ambient pressure show that the molar contents of CaSO 4 and CaO both increase with temperature, and the content of CaS diminishes accordingly. However, an exception is that at 950 °C, the conversion of CaS to CaSO 4 by FTIR analyses is lower than that at 900 °C due to the sintering of the product CaSO 4 and the change of the mechanism at higher temperatures. The higher the pressure (1.0 MPa), the more molar contents of CaSO 4 and CaO in the products can be attained, which is more pronounced at lower temperatures. The experimental results indicate that the FTIR technique can be applied to the oxidation of CaS experiments effectively to obtain the contents of the solid products.