Methanogenesis in microbes is a form of anaerobic respiration. Methanogens do not use oxygen to respire; in fact, oxygen inhibits the growth of methanogens.  The terminal electron acceptor in methanogenesis is not oxygen, but carbon. The two best described pathways involve the use of acetic acid or inorganic carbon dioxide as terminal electron acceptors:
:CO + 4 H → CH + 2 HO
:CHCOOH → CH + CO
During anaerobic respiration of carbohydrates, H and acetate are formed in a ratio of 2:1 or lower, so H contributes only  to methanogenesis, with acetate contributing the greater proportion. In some circumstances, for instance in the rumen, where acetate is largely absorbed into the bloodstream of the host, the contribution of H to methanogenesis is greater.
However, depending on pH and temperature, methanogenesis has been shown to use carbon from other small organic compounds, such as formic acid (formate), methanol, methylamines, tetramethylammonium, dimethyl sulfide, and methanethiol.  The catabolism of the methyl compounds is mediated by methyl transferases to give methyl coenzyme M.