Microbial degradation of organic material in methanogenic ecosystems is a multistep process in which subsequent groups use the products of the first groups of organisms in the chain as substrates. The acetogenic bacteria in these systems produce both H₂ and acetate. In the present minireview a thermodynamic approach is taken to evaluate the logic behind this duality. The evaluation shows that at the H₂ partial pressures that usually occur in methanogenic ecosystems the acetogenic oxidation of known acetogenic substrates such as propionate, butyrate, and benzoate yields more energy than their complete oxidation to H₂/CO₂. Also, H₂ partial pressures needed to achieve complete hydrogenogenic oxidation of these acetogenic substrates would have to be so low that H₂ would be virtually unavailable to the hydrogenotrophic bacteria, in casu the methanogens.