Experiments on acetotrophic methanogenesis were conducted at 37 degrees C in a 3.5 l chemostat reactor and ten 120 ml batch reactors. The degradation process produced substantial quantities of soluble microbial products (SMP). Converting 1 mg of acetate produced 0.0251 mg-C of SMP in the chemostat reactor but 0.0150 mg-C of SMP In batch reactors. The reaction was not pH sensitive within the range of pH 6-8. At pH 5 and 9, methanogenesis and SMP production were both inhibited. The biomass inhibited at pH 9 was able to regain activities after adjusting the pH to neutrality, but that inhibited at pH 5 was not. The biomass yield was 0.0257 and 0.0297 mg-biomass/mg-acetate in chemostat and batch reactors, respectively.