Industrial application of biomethanation is impeded by the slow development of cost effective technologies, which can supply the methanogenic archaea with H-2 both effectively and at sufficient rates in large scale. This paper is the first to present developmental work of a H-2 mass transfer technology in full-scale, investigating the H-2 mass transfer potential of a venturi-type injector through injections of up to 33.5 m(3) H-2 into a 1200 m(3) anaerobic digester under normal operation. The venturi system poses a potentially inexpensive system of in situ biogas upgrading, since it is easily installed on already existing anaerobic digesters. Experimental H-2 consumption rates increased with H-2 injection rates, and was clearly indicated to be limited by the gas-liquid mass transfer rate due to injection of large bubbles. Consumption of the unconverted H-2 was increased by recirculation of the headspace gas, but at lower rates due to dilution of H-2 with biogas. The incomplete H-2 consumption gives the venturi system application in combination with ex situ methanation systems in its present form, but the system must be developed further in order to provide sufficient gas-liquid mass transfer efficiency in order to comprise a stand-alone biogas upgrading system.