Deactivation of a 5 wt% Pd/Al2O3 catalyst during hydrogenation of citral (1) to citronellal (2), 3,7-dimethyl-2-octenal (3), and dihydrocitronellal (4) has been studied in a continuous-flow fixed-bed reactor. The reactions were carried out at 40 degreesC and 190 bar in hexane, "supercritical" CO2, or ethane as solvents. ATR-IR spectroscopic analysis of the solid/liquid interphase under reaction conditions at low pressure revealed that 1 and 3 decarbonylate on the Pd surface resulting in strongly adsorbed CO and CxHy-type hydrocarbon fragments. The absence of decarbonylation products from 2 and 4 and the excellent chemoselectivity for the hydrogenation of the C=C bonds are interpreted by adsorption of the molecules in a tilted position on Pd, pi-bonded via one C=C bond (2) or di-pi-bonded via the C=C-C=O fragment (1 and 3). A practically important observation is that a short reoxidation by air of the used Pd/Al2O3 under very mild conditions (40 degreesC, 6 bar) can circumvent further deactivation, but the original activity cannot be regained. All these observations indicate that the irreversible deactivation of Pd/Al2O3 cannot be traced to CO poisoning. We propose that site blocking caused by heavier oligomeric surface products is the major reason for the observed catalyst deactivation. (C) 2004 Elsevier Inc. All rights reserved.