The objective of this study was to investigate the effect of various flow configurations on the performance of a gas-liquid fixed bed catalytic reactor. From a theoretical point of view, the comparison is made on the basis of the different hydrodynamic and mass transfer parameters. A model for this type of reactor has been developed as a tool to determine the way in which each one of these parameters affects the reactor performance. Experimentally, a comparison of the performances obtained with a cocurrent upflow or downflow pilot unit has been done. The selective hydrogenation of butadiene in a C4 cut has been used for this comparison. The obtained selectivity is lower for the downflow than for the upflow arrangement, especially for the lower values of the liquid velocity. It is shown that the degradation in the selectivity is mainly due to problems of liquid distribution across the bed. In the case of trickling flow the problems of liquid distribution are related to the structure of the flow, in which the liquid flows in the form of rivulets with zones of the bed having an imperfect irrigation. A hysteretic behaviour in the cocurrent downflow arrangement has also been observed. (author).