Sulfate concentrations and sulfur isotopic compositions were measured in rainwater in Guiyang city in Southwest China between October 2008 and September 2009 to identify sulfur sources and their impacts on sulfur isotopic composition. The δ³⁴S values of 1235 samples collected during this period ranged from − 12.0 to + 9.4‰, with a volume-weighted mean of − 2.8 ± 9.8‰, suggesting that rainwater sulfate in Guiyang was mainly derived from SO₂ produced during coal combustion. The δ³⁴S values of rainwater sulfate increased from 1987 to 2009, with an increment of about + 0.16‰ per year, reflecting a gradual reduction in ³⁴S-depleted SO₂emitted during coal combustion. Seasonal variations in δ³⁴S values were pronounced, with higher values in winter than in summer. Long-distance transport of SO₂ from coal burned in northern cities (which was higher than the component from southern cities), and more importantly, the aqueous oxidation of SO₂ during transport affected the seasonal variation of rainwater δ³⁴S values in Guiyang. Temperature-dependent aqueous oxidation of SO₂ suggested that lower rainwater δ³⁴S coincides with higher temperature.