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The sulfur and oxygen isotope effects associated with anaerobic bacterial disproportionation of elemental sulfur by a pure culture (Desulfocapsa thiozymogenes) and an enrichment culture were investigated experimentally in the presence of synthetic Mn(IV)oxides. During bacterial disproportionation, ³⁴S/³²S were fractionated in dissolved sulfate compared to elemental sulfur by −0.6 to +2.0‰ (D. thiozymogenes) and −0.2 to +1.1‰ (enrichment culture) at cellular sulfur disproportionation rates of 10⁻¹⁶ mol S°/cell/h and 10⁻¹⁷ mol S°/cell/h, respectively. The measured sulfur isotope effects are much smaller than those observed previously for the same cultures in the presence of Fe(III) and Fe(II) compounds, indicating that microbial isotope fractionation was superimposed by the chemical re-oxidation of hydrogen sulfide by MnO₂ to sulfate. Significant re-oxidation of H₂S to sulfate was additionally confirmed by the …