In order to remove tars from a synthesis-gas produced with an allothermal, pressurized fluidized-bed biomass gasifier, a catalytic gas-cleaning-process is used. The tars are removed by means of catalytic steam reforming on a commercial nickel catalyst at temperatures of around 500 degrees C in a packed bed reactor. The synthesis-gas enters the reforming reactor at 300 degrees C. Due to the strongly exothermic methanationreaction the catalyst bed heats up to temperatures high enough to achieve sufficient catalyst activity for tar reforming. Ni-catalysts are sensitive for sulfur poisoning, therefore the main sulfur component in the hydrogen-rich synthesis-gas, hydrogen sulfide (H2S), is adsorbed in a packed bed reactor, filled with ZnO, upstream the tar reformer. Beside H2S there are further sulfur compounds in the gas stream which cannot be removed efficiently by means of ZnO. The aim of this work is to investigate whether it is possible to use a commercially available Co MO catalyst for hydrodesulfurization (HDS) upstream the ZnO-bed in order to convert organic sulfur compounds into H2S. Hydrodesulfurization processes are state-of-the-art in large-scale petrochemistry. The process-conditions in this common use-case differ from these in the application discussed within this work. To examine the ability of HDS in the process discussed here, specific HDS-tests and overall system tests were carried out with a synthesis-gas from a laboratory gasifier. The used synthesis-gas was produced with an allothermal fluidized bed gasifier with wood pellets as fuel. The HDS-catalyst started to show activity for hydrogenation of thiophene at a temperature of 350 degrees C under atmospheric pressure. The use of hydrodesulfurization showed a positive influence on the catalyst deactivation of the subsequent Ni-catalyst. (C) 2014 Elsevier B.V. All rights reserved.