The laboratory volumetric apparatus designed for the study of anaerobic decomposition of organic compounds was tested with a heterogeneous culture of microorganisms. Methanol, sodium acetate, and acetic acid were tested at cca 35^oC, 3.5-7 mmol.l^-1 organic substrate, and 2.5 g.l^-1 dry biomass. Very good reproducibility of determination was obtained for the amounts of biogas corresponding to 60-90 % theoretical methane depending of the type of substrate. Optimization of the measurement can give a substance to this method which is less expensive than the manometric method.