Sulfate aerosol is a major fraction of haze, playing an important role in aerosol formation and aging processes. In order to understand the mechanism of sulfate formations, the characteristics of sulfur isotope composition were determined during different heterogeneous oxidation reactions of sulfur dioxide. Although NH<sub>3</sub> was more beneficial to the formation of sulfate, compared with NO<sub>X</sub> and O<sub>3</sub>, <sup>34</sup>S tended to enrich the lighter sulfur isotopes in the presence of NH<sub>3</sub>. Furthermore, in consideration of the potential competitive effects of NO<sub>X</sub>, O<sub>3</sub>, and NH<sub>3</sub> in the heterogeneous oxidation processes, the contributions of each gas were evaluated via Rayleigh distillation model. Notably, NO<sub>X</sub> oxidation contributed 67.5±10 % of the whole sulfate production, which is higher than O<sub>3</sub> (13.3±10 %), and NH<sub>3</sub> oxidation (19.2±10 %) on the basic of the average fractionation factor. The observed δ<sup>34</sup>S values of sulfate aerosols were negatively correlated with sulfur oxidation ratios, owing to the sulfur isotopic fractionations during the sulfate formation processes. Given the isotope mass balance, the overall δ<sup>34</sup>S<sub>sulfate</sub> approached the δ<sup>34</sup>S<sub>emission</sub> as oxidation of SO<sub>2</sub> progressed, suggesting that NO<sub>X</sub> played a major rather than a sole role in the different heterogeneous oxidation processes of SO<sub>2</sub>.