The present work focuses on the impacts of different combustion parameters on the formation of sulfur trioxide (SO3). The outlet SO3 concentrations from a quartz reactor operated within the temperature range of 800 K to 1673 K were measured using the controlled condensation method. Post-flame conditions were examined with and without combustibles, and the effects of SO2, O-2, NO, H2O, and CO2 on SO3 formation were investigated. The formation of SO3 along the quartz reactor was modeled with a detailed chemical reaction mechanism by assuming plug-flow and using measured temperature profiles. Only reactions that occurred in the gas phase were considered. In the absence of combustibles, the outlet SO3 concentration increased as the experimental temperature and O-2 concentration increased. When reactive gases (e.g., NO, CO, and CH4) were introduced, the formation of SO3 was increased, mainly as the result of increased concentrations of radicals. In addition, the combustion atmosphere (comprising N-2, CO2, and H2O) influenced the amount of SO3 formed. A higher concentration of H2O clearly increased SO3 formation in the absence of combustibles. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.