Desk studies were carried out to calculate the feasibility of hydrogen gas production by (hyper)thermophilic organisms in high rate bioreactors. One of the main problems to deal with is the requirement of a hydrogen gas pressure lower than 20, 000 Pa. Only under these conditions carbohydrates are converted into hydrogen gas, carbon dioxide and acetic acid. These conditions can be created by stripping the hydrogen gas with steam produced by evaporation of water at a large surface area created by packing material in an anaerobic trickling filter. The steam production occurs at 70degreesC and an absolute pressure of 0.5 bar. In these reactors gas is the continuous phase and the substrate solution is continuously trickled over the packing material, which also supports biofilms of (hyper)thermophilic bacteria. In this way hydrogen producing biomass, the liquor with substrate and the gas phase are within millimetres reach of each other. To compensate for the heat loss by water evaporation, a part of the water and heat is recovered after gas compression. The remaining is recovered from steam produced by the fuel cell in which the hydrogen gas is converted into electricity. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.