Isotope fractionations during sulfate reduction by natural bacterial populations were measured in seven different marine sediments and compared with the isotopic composition of solid-phase sulfides in the same sediments. The measured fractionations during sulfate reduction could explain only between 41% and 85% of the 34S depletion in the sedimentary sulfides. This result directly demonstrates that the depletion of 34S in solid sulfides is an expression of the combined activity of sulfate-reducing organisms with additional fractionations accumulated during the oxidative part of the sulfur cycle. The only known process to significantly augment the fractionations created during sulfate reduction is the microbial disproportionation of the intermediate sulfur compounds: elemental sulfur, thiosulfate, and sulfite. In a simple model, we show how important each of these disproportionation processes could be in generating the isotopic composition of sedimentary sulfides. The sulfate reduction rates in the sediments studied varied by a factor of 1000 and did not correlate with the fractionation during sulfate reduction. By contrast, a correlation was observed between sulfate- reduction rate and the extent of 34S depletion into sedimentary sulfides, the most 34S-depleted sulfides found in sediments supporting the lowest rates of sulfate reduction. Thus, the fractionations imposed during the disproportionating processes are better expressed in sediments with low sulfate-reduction rates. The direct phototrophic oxidation of sulfide to sulfate, with minimal fractionation, may have been important in the sediments that had a high sulfate-reduction rate.