Whole-cell and cell-extract experiments were performed to study the mechanism of oxalate metabolism in the acetogenic bacterium Moorella thermoacetica. In short-term, whole-cell assays, oxalate consumption was low unless cell suspensions were supplemented with CO2, KNO3, or Na2S2O3. Cell extracts catalyzed the oxalate-dependent reduction of benzyl viologen. Oxalate consumption occurred concomitant to benzyl viologen reduction; when benzyl viologen was omitted, oxalate was not appreciably consumed. Based on benzyl viologen reduction, specific activities of extracts averaged 0.6 μmol oxalate oxidized min−1 mg protein−1. Extracts also catalyzed the formate-dependent reduction of NADP+; however, oxalate-dependent reduction of NADP+ was negligible. Oxalate- or formate-dependent reduction of NAD+ was not observed. Addition of coenzyme A (CoA), acetyl-CoA, or succinyl-CoA to the assay had a minimal effect on the oxalate-dependent reduction of benzyl viologen. These results suggest that oxalate metabolism by M. thermoacetica requires a utilizable electron acceptor and that CoA-level intermediates are not involved.