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Riverine dissolved sulfate (SO 4 2- ) sulfur and oxygen isotope variations reflect their controls such as SO 4 2- reduction and reoxidation, and source mixing. However, unconstrained temporal variability of riverine SO 4 2- isotope compositions due to short sampling durations may lead to mischaracterization of SO 4 O) of Mississippi River SO 2- sources, particularly for the pyrite-derived sulfate load. We measured the sulfur and triple-oxygen isotopes (δ 34 S, δ 18 O ranged from -6.3‰ to -0.2‰ and -3.6‰ to +8.8‰, respectively. Our sampling period captured the most severe flooding and drought in the Mississippi River basin since 1927 and 1956, respectively, and a first year of sampling that was unrepresentative of long-term average SO 17 O) of Mississippi River SO 4 2- with biweekly sampling between 2009 and 2013 to test isotopic variability and constrain sources. Sulfate δ 34 sources are 74 ± 10% of the Mississippi River sulfate budget. Furthermore, pyrite oxidation is implicated as the dominant process supplying SO 18 to the Mississippi River, whereas the Δ' 4 2- . The δ 34 S SO4 data indicate pyrite-derived SO 4 2- sources are 74 ± 10% of the Mississippi River sulfate budget. Furthermore, pyrite oxidation is implicated as the dominant process supplying SO 4 2- to the Mississippi River, whereas the Δ' 17 O SO4 data shows 18 ± 9% of oxygen in this sulfate is sourced from air O 2 .