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In the present study, coupled stable sulfur and oxygen isotope fractionation during elemental sulfur disproportionation according to the overall reaction: 4H2O + 4Sˆ → 3H2S + SO4 2 − + 2H+, was experimentally investigated for the first time using a pure culture of the sulfate reducer Desulfobulbus propionicus at 35ˆC. Bacterial disproportionation of elemental sulfur is an important process in the sulfur cycle of natural surface sediments and leads to the simultaneous formation of sulfide and sulfate. A dual-isotope approach considering both sulfur and oxygen isotope discrimination has been shown to be most effective in evaluating specific microbial reactions. The influence of iron- and manganese bearing-solids (Fe(II)CO3, Fe(III)OOH, Mn(IV)O2) acting in natural sediments as scavengers for hydrogen sulfide, was considered, too. Disproportionation of elemental sulfur was observed in the presence of iron solids at a cell-specific sulfur disproportionation rate of about 10− 9.5± 0.4 μ mol Sˆ cell− 1 h− 1. No disproportionation, however, was observed with MnO2. In the presence of iron solids, newly formed sulfate was enriched in 18 O compared to water by about +21‰ (≡ ϵ H2O ), in agreement with a suggested oxygen isotope exchange via traces of intra- or extracellular sulfite that is formed as a disproportionation intermediate. Dissolved sulfate was also enriched in 34S compared to elemental sulfur by up to +35%. Isotope fractionation by Desulfobulbus propionicusis highest for all disproportionating bacteria investigated, so far, and may impact on the development of isotope signals at the redox boundary of surface sediments. Keywords: bacterial disproportionationelemental sulfurstable isotope fractionation Acknowledgments MEB wishes to thank Jürgen Rullkötter and Hans Brumsack for the possibility to use the mass spectrometer and ICP-OES, respectively, at the ICBM, University of Oldenburg. Swantje Lilienthal is thanked for technical assistance. FTIR spectroscopic measurements were carried out by Peer-L. Gehlken. The study was supported by Max Planck Society, Munich. The constructive comments of two reviewers are highly acknowledged. Notes 1 CitationBöttcher et al. (2001) and CitationBöttcher and Thamdrup (2001). 2This study. 3 CitationCanfield et al. (1998). 4 CitationTheune et al. (2005); CitationSmith (2000).