Several recent studies have shown the potential of oxy-fuel combustion to reduce NO
(gas) and SO
(gas) emissions. However, the mechanisms through which SO
(gas) reduction takes place has yet to be fully understood. Therefore, the development of oxy-sulfate thermodynamic database for a better understanding and control of SO
(gas) emission during oxy-fuel combustion processes is essential. The focus of this research is on the thermodynamic modelling of the iron oxide–sulfate system with the FactSage 7.2 software package. Thermodynamic properties of selected phases in the FeO
system were critically reviewed, compiled and assessed over a wide temperature range (298–2000 K) to obtain accurate thermodynamic description of the system at different temperatures. New C
functions, which include the recent experimental data, were optimized. The obtained results are presented and discussed.