H2O concentration in an oxy-fuel furnace is higher than that in a conventional air combustion furnace. Pyrite is a dominant factor that contributes to slagging in pulverized coal-fired furnace. This study investigated the effect of H2O on pyrite transformation in the presence or absence of O-2 atmosphere. The TGA with a Netzsch STA449F3 thermobalance using a water vapor furnace was applied in the study. The results showed that the onset temperature T-o and the end temperature T-e of pyrite decomposition decreased as H2O partial pressure increased in O-2-free atmospheres. Higher H2O concentration promoted the product iron sulfide of lower sulfur content pyrrhotite FeSx (here 1 < x < 2), and magnetite transformed into hematite in O-2-free atmospheres. The thermal decomposition temperature of pyrite is lower in H2O than that in CO2 atmosphere. In H2O atmosphere, the reaction between pyrrhotite and H2O may form magnetite, H2S and SO2 during the thermal decomposition of the pyrite. As H2O concentration increased, pyrite conversion and the release of sulfur were increased in CO2 atmosphere. In O-2-containing atmospheres, the presence of H2O improved the initial transformation of pyrite, and an increase in O-2 concentration also achieved similar results. However, the overall effect of H2O concentration was weak in O-2-containing atmospheres. A comprehensive pathway of pyrite transformation during oxy-fuel combustion was proposed. (C) 2014 Elsevier B.V. All rights reserved.