The influence of oxygen and nitric oxide on the reactions of sulfur dioxide with ammonia were studied in a simulated flue gas in the range of 0-20% oxygen and 0-300 ppm nitric oxide at temperatures in the range of 40-60 degrees C. A Fourier transform infrared spectrometer (FT-IR) analyzed the reaction products deposited on the reactor surface and revealed that ammonium sulfate was the main product of the reactions, with sulfamic acid and ammonium sulfamate as the minor products. The results showed that oxygen and nitric oxide enhanced the oxidation reactions of sulfur dioxide to form ammonium sulfate. The yield of the minor products markedly increased in the presence of nitrogen dioxide. The size and number concentration of product aerosols increased at lower temperature. The fraction of sulfur dioxide which formed aerosols increased with sulfur dioxide removal.