Isotope exchange reactions between 35S-labeled sulfur compounds were studied in anoxic estuarine sediment slurries at 21°C and pH 7.4–7.7. Two experiments labeled with radioactive elemental sulfur ( 35S°) and one labeled with radioactive sulfate ( 35SO 4 2−) were performed as time-course experiments of up to 10 days. More than 85% of the added 35S° was distributed into the reduced sulfur pools comprising ΣHS −(=H 2S-HS −+S 2−), iron sulfide (FeS) and pyrite (FeS 2) in less than 10 min after 35S° labeling. When 1.2 μmol ΣHS − cm −3 was present in the slurry, 64% of the total 35S was recovered in the ΣHS − pool in less than 1.5 h. With no detectable σHS − (less than 1 μM) in the slurry, 58% of the total 35S was observed in the pyrite pool within 1.5 h. The FeS pool received up to 31% of all 35S added. The rapid 35S incorporation from 35S° into ΣHS − and FeS pools was explained by isotope exchange reactions. In contrast, there was evidence that the radioactivity observed in the ‘pyrite pool’ was caused by adhesion of the added 35S° to the FeS 2 grains. In all 35S°-labeled experiments we also observed oxidation, in the absence of oxygen, of reduced sulfur to 35SO 4 2−, which reached more than 24% of the total 35S after 72 h. In the 35SO 4 2− experiment, ΣH 35S − was formed and we observed a significant isotope exchange between H 35S − and S° as well as precipitation of Fe 35S. As a result of isotope exchange, specific radioactivities of the reduced sulfur pools were poorly defined and could not be used to calculate their rates of formation. Such isotope exchange reactions between the reduced inorganic sulfur compounds will affect the stable isotope distribution and are expected to decrease the isotopic fractionations. All 35S, however, that was incorporated into AVS (i.e. ΣHS −+FeS), S° and FeS 2 during short-term 35SO 4 2− incubations originated from the formed H 2 35S, and realistic sulfate reduction rates therefore were measured from the sum of the radioactivities of reduced sulfur compounds.