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.