The 34S-to-32S ratio in dissolved SO4 has been studied in the Kalix River, Northern Sweden, and its catchment. Weekly sampling over 17 months revealed temporal variations from +5.3‰ up to +7.4‰ in the δ34S values in the river. Snow and rain samples showed lower δ34S values (average +5.6‰ and +5.0‰, respectively). The atmosphere is the major source for S in surface waters in the catchment, and the heavier δ34S values in the river are a result of SO4 reduction within the catchment. Most of the temporal variations in the δ34S value in the river are caused by a mixing of water from the mountain areas (relatively light δ34S) and the woodland. The δ34S value is relatively heavy in the woodland tributaries because of bacterial SO4 reduction in peatland areas influenced by groundwater. The highest δ34S values were measured during the spring flood, in June and in November. These heavy δ34S values are related to different types of water with diverse origins. The heavy δ34S values coinciding with the early spring flood originate from peatland areas in the woodland. Relatively heavy δ34S values (up to +14.4‰) were registered in mire water. Smaller variations of the δ34S value during summer and early autumn most likely were caused by the input of ground-mire water during heavy rains. A correlation between increased TOC concentrations and increased δ34S values was observed. The heavy δ34S values in June and November probably originate from SO4 reduction in bottom water and sediments in lakes within the catchment. Bottom water, enriched in 34S---SO4, was transported in the river during the spring and autumn overturn.