BACKGROUND With high surface-to-volume ratios, hollow fibre membranes offer a potential solution to improving gas-liquid mass transfer. This work experimentally determined the mass transfer characteristics of commercially available microporous hollow fibre membranes and compared these with the mass transfer from bubble column reactors. Both mass transfer systems are considered for biological methanization, a process that faces a challenge to enhance the H-2 gas-liquid mass transfer for methanogenic Archaea to combine H-2 and CO2 into CH4. RESULTS Polypropylene membranes showed the highest mass transfer rate of membranes tested, with a mass transfer coefficient for H-2 measured as k(L) = 1.2 x 10(-4) ms(-1). These results support the two-film gas-liquid mass transfer theory, with higher mass transfer rates measured with an increase in liquid flow velocity across the membrane. Despite the higher mass transfer rate from polypropylene membranes and with a liquid flow across the membrane, a volumetric surface area of alpha = 10.34 m(-1) would be required in a full-scale in situ biological methanization process with much larger values potentially required for high-rate ex situ systems. CONCLUSIONS The large surface area of hollow fibre membranes required for H-2 mass transfer and issues of fouling and replacement costs of membranes are challenges for hollow fibre membranes in large-scale biological methanization reactors. Provided that the initial bubble size is small enough (d(e) < 0.5 mm), calculations indicate that microbubbles could offer a simpler means of transferring the required H-2 into the liquid phase at a head typical of that found in commercial-scale anaerobic digesters. (c) 2019 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.