Bubbleless gas transfer through a hollow fiber membrane (HFM) module was used to supply H-2 to an anaerobic reactor for in situ biogas upgrading, and it creates a novel system that could achieve a CH4 content higher than 90 % in the biogas. The increase of CH4 content and pH, and the decrease of bicarbonate concentration were related with the increase of the H-2 flow rate. The CH4 content increased from 78.4 % to 90.2 % with the increase of the H-2 flow rate from 930 to 1,440 ml/(l day), while the pH in the reactor remained below 8.0. An even higher CH4 content (96.1 %) was achieved when the H-2 flow rate was increased to 1,760 ml/(l day); however, the pH increased to around 8.3 due to bicarbonate consumption which hampered the anaerobic process. The biofilm formed on the HFM was found not to be beneficial for the process since it increased the resistance of H-2 diffusion to the liquid. The study also demonstrated that the biofilm formed on the membrane only contributed 22-36 % to the H-2 consumption, while most of the H-2 was consumed by the microorganisms in the liquid phase.