Because SO3 participates in both high- and low-temperature corrosion processes, there is a general concern about the SO3 formation under oxy-fuel fired conditions. This work has the aim to evaluate the influence of combustion parameters on the formation of SO3. Experiments were conducted in oxy-fuel and air-fired experiments with propane as fuel and injection of SO2 in the oxidizer. The SO3 concentration was measured with a controlled condensation method at the furnace outlet as well as in the flame. The experiments show that the gas-phase is an important contributor to SO3 formation and that the SO3 formation is strong during burnout of the fuel. In oxy-fuel combustion with wet flue-gas recycle (FGR), more SO3 was formed than during dry FGR at similar temperature conditions, which indicates that H2O enhances SO3 formation. The experiments also show that the SO3 formation rises with an increase in furnace temperature. Because temperature and residence time in the furnace increases with reduced FGR ratio, the FGR ratio directly influences the SO3 formation in oxy-fuel combustion. This was obvious during the experiments, and the SO3 concentration rose with a reduced FGR ratio.