The present paper was addressed to mineral transformations and ash formation during O-2/CO2 combustion of pulverized coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions. The ash samples were characterized with XRD analysis and Fe-57 Mossbauer spectroscopy. The impacts Of O-2/CO2 combustion on mineral transformation and ash formation were explored through comparisons between O-2/CO2 combustion and O-2/N-2 combustion. It was found that, O-2/CO2 combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviors of included minerals. (c) 2007 Elsevier Ltd. All rights reserved.