Sulfur trioxide in gas turbine exhaust contributes to particulate emissions; reduction of this compound is a means for control of particulate emissions. The chemical kinetics of SO3 formation were analyzed for a large stationary gas turbine. The source of SO3 is the reaction of SO2 with oxygen atoms in the downstream end of the combustor primary zone. The primary zone produces SO3 levels of 1 to 2 percent of total SOx. SO3 increases above 1 to 2 percent during air dilution from an equivalence ratio of about 0.5 to 0.35; formation times are on the order of 1 to 10 ms. Reduction of primary zone air flow and more rapid dilution air mixing were identified as means for SO3 reduction. Dilution air mixing in 1 ms was identified as an objective; but would be a difficult task with current combustor designs.