In this work, it has been investigated a membrane-like effect of ionic liquid (IL) in the multistep catalytic hydrogenation of trans-cinnamaldehyde (CALD) and 1,5-ciclooctadiene (1,5-COD). A commercial Pd/C catalyst was impregnated with 1-hexyl-3-methyl-imidazolium at 0.5-10 wt%. SEM, EDS, XRD, TG and BET data suggested that the IL penetrates the carbon pore structure coating the catalyst, especially at 5 and 10% concentration. The presence of the IL layer strongly affected the selectivities of the hydrogenations of CALD and 1,5-COD. In the case of CALD, the IL favored the formation of the fully hydrogenated product (hydrocinnamyl alcohol). This result is discussed in terms of a membrane-like effect, in which the small H-2 molecule diffuses more easily from toluene through the IL layer as compared to the larger cinnamaldehyde. This effect results in the production of comparatively higher concentrations of the Pd-H species, which favors the double hydrogenation. On the other hand, the IL layer during the 1,5-COD hydrogenation favored the intermediate product COE (cyclooctene). In this case, a membrane-like effect was also used to discuss the result, where the IL layer has the effect to expel the intermediate COE hindering its contact with the Pd surface. (C) 2012 Elsevier B.V. All rights reserved.