Fly ash plays an important role in the retention and oxidation of SOx species in the coal flue gas stream, particularly in the oxy-combustion of coal where the sulfur trioxide (SO3) concentration is of great concern. Density functional theory was employed to examine the binding mechanism of SO3 to several of the various metal oxides that compose coal fly ash, including calcium oxide, magnesium oxide, sodium oxide and potassium oxide, to better understand the interactions between the gaseous SO3 and the metal oxide surfaces. Results from the calculations show the trend that SO3 binds more strongly to Mg < Ca < Na < K through the formation of a sulfate (SO42-)-like molecule. (C) 2015 Elsevier Ltd. All rights reserved.