Acetobacterium woodii and Eubacterium limosum were grown in a defined medium or suspended in buffer containing syringate as the sole organic carbon source to test whether H-2 would support acetogenesis from a phenylmethylether and CO2. When growing and resting cell preparations were incubated under various headspace gas compositions, consistent O-demethylation activity and growth were observed on syringate-CO2 when H-2 served as the sole energy source. However, the dependency for H-2 was relieved by addition of yeast extract. A hypothetical scheme representing hydrogenolytic (reductive) O-demethylation and mixotrophic acetogenesis is proposed in light of these results and other literature observations.