This investigation examines the effectiveness of a silicone rubber membrane to separate biogas from the liquid medium in the hydrogen fermentation reactor. When the culture liquid circulates through the shell side of the hollow fibers, the biogas diffuses into the lumen of the hollow fibers and is removed by a vacuum pump. When the cross flow velocity along the module increases from 0.33 to 1.19 cm/s, the CO2 mass transfer coefficient increases by a factor of 3.9, from 0.81 x 10(-7) to 3.19 x 10(-7) cm/s kPa, and the H(2)mass transfer coefficient increases by a factor of 3.5, from 0.55 x 10(-8) to 1.96 x 10(-8) cm/s kPa. The mass transfer coefficients Of CO2 and H-2 are directly proportional to the cross flow velocity, and the values for CO2 exceed those for H-2. However, silicone rubber effectively reduces the biogas partial pressure in the hydrogen fermentation reactor, and improves the hydrogen evolution rate by 10% and the hydrogen yield by 15%. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.