<p>Acetate is an important intermediate during the degradation of organic matter in anoxic flooded soils and sediments. Acetate is disproportionated to CH<sub>4</sub> and CO<sub>2</sub> by methanogenic or is oxidized to CO<sub>2</sub> by sulfate-reducing microorganisms. These reactions result in carbon isotope fractionation, depending on the microbial species and their particular carbon metabolism. To learn more about the magnitude of the isotopic enrichment factors (ε) involved, acetate conversion to CH<sub>4</sub> and CO<sub>2</sub> was measured in anoxic paddy soils from Vercelli (Italy) and the International Rice Research Institute (IRRI, the Philippines) and in anoxic lake sediments from the north east (NE) and the south west (SW) basins of Fuchskuhle (Germany). Acetate consumption was measured using samples of paddy soil or lake sediment suspended in water or in phosphate buffer (pH 7.0), both in the absence and presence of sulfate (gypsum), and of methyl fluoride (CH<sub>3</sub>F), an inhibitor of aceticlastic methanogenesis. Under methanogenic conditions, values of ε<sub>ac</sub> for acetate consumption were always in a range of -21 ‰ to -17 ‰, but higher in the lake sediment from the SW basin (-11 ‰). Under sulfidogenic conditions εac values tended to be slightly lower (-26 ‰ to -19 ‰) especially when aceticlastic methanogenesis was inhibited. Again, ε<sub>ac</sub> in the lake sediment of the SW basin was higher (-18 ‰ to -14 ‰). Determination of ε<sub>CH4</sub> from the accumulation of <sup>13</sup>C in CH<sub>4</sub> resulted in much lower values (-37 ‰ to -27 ‰) than from the depletion of <sup>13</sup>C in acetate (-21 ‰ to -17 ‰), especially when acetate degradation was measured in buffer suspensions. The microbial communities were characterized by sequencing the bacterial 16S rRNA genes as well as the methanogenic <em>mcrA</em> and sulfidogenic <em>dsrB</em> genes. The microbial communities were quite different between lake sediments and paddy soils, but were similar in the sediments of the two lake basins and in the soils from Vercelli and IRR, and were similar after preincubation without and with addition of sulfate (gypsum). The different microbial compositions could hardly serve for the prediction of the magnitude of enrichment factors.</p>