This study aimed at studying the biomethanation process using a 100 L pilot-scale digester equipped with a dense membrane for hydrogen injection. Hydrogen mass transfer was characterized and the impact of hydrogen flowrate, agitation rate and of the co-injection of CO2, on biogas production and composition, was precisely studied. A linear relationship between H-2 flowrate and the CO2 and CH4 rates in biogas was found but no impact on biogas flowrate was shown. It was also noticed that, without exogenous CO2 injection, and for high H-2 injection flowrates, residual H(2 )could be found at the digester outlet due to local CO2 limitation. Thus, this study suggested that biogas production in biomethanation process at the pilot scale was probably rather limited by the dissolved CO2 transport within the liquid phase than by the hydrogen mass transfer itself.