The performance of trickle- and flooded-bed reactors has been investigated and compared for an exothermic multi-step catalytic reaction. Selective hydrogenation of cyclododecatriene over Pd/Al2O3 has been studied in both up-and down-flow modes of operation in the same pilot reactor. In the down-flow mode, hot spots and runaway could not be avoided without diluting both catalyst bud and liquid reactant. With this diluted system, the up-flow reactor leads to a higher productivity and a much better selectivity. A non-isothermal plug-flow reactor model predicts the performances of the up-Row reactor satisfactorily, but is found to be unsuitable to the case of a trickle-bed reactor. In the latter case, the productivity was underestimated, when complete wetting of catalyst particles was assumed. On the other hand, when partial wetting effect was incorporated, the calculated selectivity was always much higher than that observed actually in a trickle bed, due to heterogeneities of liquid velocity and partial wetting (poorly irrigated zones).