Acetate is one of the most important intermediates for methanogenesis in the anaerobic mineralization of organic materials. Methanogenic acetate degradation is carried out by either an aceticlastic reaction or an anaerobic acetate-oxidizing reaction. In contrast to the former reaction, the latter is energetically extremely unfavorable. However, the oxidation of acetate can occur with syntrophic interaction between certain bacteria and methanogenic archaea. The bacteria, namely syntrophic acetate-oxidizing bacteria, can oxidize acetate to produce hydrogen/CO₂ only when their products are subsequently utilized by the hydrogen-scavenging methanogens. Surprisingly, some of these bacteria can also axenically grow on hydrogen/CO₂ to produce acetate. This means that the bacteria can utilize both substrates and products reversibly. This review describes current studies of these curious and fascinating microbes.