Biogas is a clean environment friendly fuel that is produced by bacterial conversion of organic matter under anaerobic (oxygen-free) conditions. Raw biogas contains about 55-65% methane (CH4), 30-45% carbon dioxide (CO2), traces of hydrogen sulphide (H2S) and fractions of water vapour. Pure methane has a calorific value of 9100 kcal/m(3) at 15.5 degrees C and 1 atm; the calorific value of biogas varies from 4800 to 6900 kcal/m3. TO achieve the standard composition of the biogas and calorific value of 5500 kcal/m(3) the treatment techniques like absorption or membrane separation should be applied. In the paper the results of the tests of the CH4 enrichment in simulated biogas mixture consisted of methane, carbon dioxide and hydrogen sulphide were presented. It was showed that using the capillary module with polyimide membranes it was possible to achieve the enrichment of CH4 from the concentrations of 55-85% up to 91-94.4%. The membrane material was resistant to the small concentrations of sour gases and assured the reduction of H2S and water vapour concentrations, as well. The required enrichment was achieved in the single module, however to prevent CH4 losses the multistage or hybrid systems should be used to improve process efficiency. (c) 2007 Elsevier B.V. All rights reserved.