Hydrogen conversion of CO by a pure culture of Carboxydothermus hydrogenoformans was investigated and optimized in a lab-scale hollow fiber membrane bioreactor (HFMBR). The reactor was operated under strict anaerobic, extremely thermophilic (70 degrees C) conditions with a continuous supply of gas, for four months. Reactor performance was evaluated under various operational conditions, such as liquid velocity (upsilon(liq)) (13, 65 and 130 m h(-1)), temperature (70, 65, and 60 degrees C), CO pressure (from 1 to 2.5 atm) and CO loading rate (from 1.3 to 16.5 mol L-rxr(-1) d(-1)). Overall, results indicated a relatively constant H-2 yield of 92 +/- 4% (mol mol(-1)) regardless of the operational condition tested. Permeation across the colonized membrane was improved by three orders of magnitude as compared to the abiotic membrane, because of dissolved CO concentration was constantly maintained low in the liquid on the shell side of the membrane as continually depleted by the microorganisms. Once the biofilm was sufficiently developed, a maximum CO conversion activity of 0.44 mol CO g(-1) volatile suspended solid (VSS) d(-1) was achieved at a p(CO) of 2 atm or above and a upsilon(liq) of 65 m h(-1). However, this highest activity represented only 15% of the maximal activity potential of the strain under non-limiting conditions, attributed to the low concentration of dissolved CO (0.01-0.07 mM) present in the HFMBR liquid. Higher upsilon(liq) (130 m h(-1)) produced shearing stress, which detached a significant portion of the biofilm from the membrane, and/or prevented less sessile growth (57% total biomass as biofilm, as opposed to 84-86% at lower upsilon(liq)). One may deduce from this work that the volumetric CO conversion performance of such a membrane bioreactor would be at the most in the range of 5 mol CO L-rxr(-1) d(-1). Overall, the CO conversion performance in the HFMBR was biokinetically limited, when not limited by gas-liquid mass transfer. Additionally, over time, membrane fouling and aging decreased membrane permeability such that the CO transfer rate would be the most limiting factor in the long run. Crown Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.