The transformation of pyrite (FeS2) is critical to ash slagging in oxy-fuel combustion but has little been explored. The chemical role of CO2, the dilution gas in oxy-fuel boilers, on pyrite decomposition was investigated in this work. Pyrite samples of 63–75 lm were decomposed at temperatures between 948 and 1073 K, and in high-purity CO2 or N2 in a novel thermo-gravimetric reactor (TGR) under isothermal conditions. Sample weight loss with respect to time was recorded online and time-resolved data on the evolution of gas products were obtained by Fourier transform infrared spectroscopy (FT-IR). The solid products were characterized by X-ray diffraction (XRD). The results show that pyrrhotite is the only iron product detected in either CO2 or N2, but the pyrrhotite from CO2 contains less sulfur than that from N2. Pyrite decomposition in CO2 is noticeably faster and has lower activation energies than that in N2. These effects of CO2 are chemical in nature and are evidenced by the evolution of SO2, CO and COS in addition to sulfur clusters. Based on the data of both solid and gas products, pyrite decomposition in either CO2 or N2 is divided into three distinct stages. However, different from N2, CO2 is found to participate in pyrite decomposition at each stage. A mechanism is proposed to account for the chemical role of CO2 in pyrite decomposition.