This study investigates SO3 formation under air-fired and oxy-fuel combustion environments in a bench-scale bubbling fluidised bed. Flue gas compositions typical of air-fired and oxy-fuel environments were simulated with synthetic gas mixtures. Parameters such as bed temperature and SO2 and H2O concentrations were varied to determine their effect on the formation of SO3. Tests were also performed using three fly ashes produced from three different coals and combustion systems. Oxy-fuel combustion led to increased SO3 concentration as compared to typical air-fired environments. SO3 concentrations increased with combustion temperature and O-2 concentrations. SO3 formation also depends on the chemical composition of the fly ash. (C) 2017 Published by Elsevier B.V.