Hydrogen is a clean energy carrier which can be used as fuel in fuel cells. Today, hydrogen is produced mainly by steam reforming of fossil fuels like natural gas or oil. But only hydrogen produced by renewable sources can be called clean energy production. One possibility for hydrogen production is the biological fermentation of biogenous wastes by hydrogen producing bacteria. For the experimental setup four 30-L-working-volume reactors were constructed for continuous biohydrogen production. As inoculum, heat-treated sludge of a wastewater treatment plant was used. Different hydraulic retention times (HRT) were tested and an organic loading rate (OLR) of 2-14 kg VS/m(3)*d. As starting substrate, waste sugar medium was used. The pH and other parameters were observed to find boundary conditions for a stable continuous process with a minimum of online-control measurements. The high concentration of organic acids in the reactor led to a very low pH, which was controlled manually and online > 4 up to 5.5, otherwise the biohydrogen production decreased rapidly. The gas amount varied with the different OLRs, but could be stabilised on a high level as well as the hydrogen concentration in the gas with 44-52%. No methane was detected in the gas. It turned out, that continuous biohydrogen production with stable gas amounts and qualities could be achieved at different operation conditions. The results showed, that the operation of a continuous biohydrogen reactor has to be observed very carefully to ensure a constant gas production, and that pH-control is necessary to ensure stable operation conditions. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.