ABSTRACT Sulfur-containing species are of central importance for atmospheric chemistry and climate change, as they significantly influence the formation and chemical composition of submicron particulate matter (PM1). While SO2 and sulfate concentrations in Lithuania have continuously decreased since 1981 it remains unclear whether the sources and transformation mechanisms of SO42− have changed. In this study, the concentrations and sulfur isotope composition of SO2 and PM1 sulfate were measured over a two-year period to identify possible sources and formation processes of sulfur-containing species in Vilnius, Lithuania. Elevated concentrations of sulfur species were observed in autumn and winter, corresponding to lower δ34S values in winter (3.1‰ for SO2 and 0.9‰ for SO42−) and higher in summer (6.5‰ for both SO2 and SO42−). Source apportionment revealed that biomass burning was the dominant contributor to sulfur pollution in summertime (66 ± 16% in SO2 and 63 ± 17% in SO42−). In contrast, coal combustion was the primary source of SO42− during the winter of 2021–2022 (79 ± 14%), while heavy fuel oil (HFO) emissions from Vilnius thermal power station dominated both sulfate (43 ± 12%) and SO2 (40 ± 16%) fractions in 2022–2023. The airflows from the south and southwest were associated with increased SO42− concentrations and lower δ34S values. In contrast, SO2 concentrations showed only minimal variations depending on the air mass origins. These results indicate that local pollution sources are the major contributors to gaseous SO2 pollution, while SO42− pollution in Vilnius is mainly associated with regional sources. This suggests that increased SO2 emissions from HFO combustion had a significant impact on sulfate formation, predominantly through the transition metal ion oxidation pathway, in the Vilnius area.