The reaction between sulfur dioxide and ammonia in flue gas is of importance in understanding scaling on process equipment and monitoring systems in a number of flue gas cleaning processes. The reaction was studied at atmospheric pressure using simulated flue gas, 1.5-12.0-mm-internal diameter tube reactors, and flows ranging from 0.5 to 4.0 (N L)/min. The reaction temperature, sulfur dioxide concentration, and ammonia concentration were 40-80 °C, 60-1000 ppm, and 120-2000 ppm, respectively. The water content of the gas was typically 6 vol % and the oxygen content 5 vol %. TTie first step of the reaction is the formation of liquidlike ammonium salt containing sulfur in oxidation state IV on surfaces. With oxygen and small amounts (5 ppm) of nitrogen dioxide present, S(IV) is irreversibly oxidized to S(VI), and the product becomes solid and thermally stable. This product contains primarily ammonium sulfate. An overall mechanism for the reaction involving two intermediary species is proposed, and a simple three-parameter expression for the reaction rate has been developed from the mechanism.