In this paper, experimental results are presented, showing the influence of the hydrodynamic behavior of fixed-bed catalytic reactors with two-phase (gas + liquid) flow on the selectivity of a reaction. A mathematical model has been developed in order to represent up-flow data, for which the distribution of phases across the catalytic bed is near ideal. The influence of maldistribution of the phases is noticeable for down-dow conditions. In this case, recommendations for the design of the reactor are given.