Sulfate (SO42-) is an important chemical species in atmospheric aerosols, which strongly impacts atmospheric chemistry processes and climate change. Stable sulfur isotopes (delta S-34) of sulfate aerosols in PM2.5 were measured in Beijing from November 13 to December 2, 2018, to investigate the pathways of formation of sulfate aerosols. The results showed that SO42- constituted a major fraction (18%) of water-soluble ions and significant enhancement of sulfate was observed during the haze period. The delta S-34-SO42- values averaged at 4.4 +/- 1.4 parts per thousand during the full period, exhibiting a downward trend with an increase in sulfate concentration. The change in sulfur isotope values could not be explained by the changes in emission sources. Significant correlations were found between observed delta S-34-SO42- values and SO2 oxidation ratios (R = -0.88; p < 0.01), indicating the changes in sulfur isotopes were attributed to the SO2 oxidation processes. On the basis of Rayleigh distillation, the average fractionation factor between SO2 and SO42- was 4.0 +/- 1.2 parts per thousand. Combining sulfur isotopes and the Bayesian model, we quantified the contributions of primary sulfate, OH, H2O2/O-3, NO2, and O-2 [catalyzed by transition metal ions (TMIs)] oxidation pathways to sulfate formation were 7%, 20%, 16%, 27%, and 30%, respectively. The contributions of TMI and NO2 pathways increased from 24% and 20% during the clean period to 38% and 29% during the haze period, respectively. Our results highlighted that sulfur dioxide oxidized by TMI-catalyzed O-2 and NO2 were the dominant pathways of sulfate formation in Beijing under haze pollution during the heating seasons.