A nonisothermal thermogravimetric analysis technique was used to evaluate the effect of CO2 concentration on the overall reactivity of Indian coal chars during CO2/O2 combustion. The study was carried out for the drop tube furnace derived chars of three Indian sub-bituminous coals from different sources. The selected coal char samples were combusted in different CO2/O2 molar volume combinations, namely, 80:20, 60:40, 40:60, 20:80, and 0:100 (pure O2) so that the effect of the concentration of CO2 on the overall reaction rate could be quantified. The Arrhenius constants (E and A) were determined using the random pore model. The concept of weighted mean activation energy was used to account for the multi-Arrhenius linearities. Empirical equations were proposed to evaluate the Arrhenius constants in terms of the mole fraction of CO2 for the overall reaction rate. © 2011 American Chemical Society.