The deactivation of a monolithic Pd/alpha-alumina catalyst in the selective hydrogenation of acetylene in the presence of excess ethylene has been studied. The deactivation behavior of the monolithic catalyst in the gas-phase hydrogenation was compared to that of the liquid-phase hydrogenation. It was shown that the deactivation behavior was different in the two systems. It was concluded that in the gas-phase hydrogenation the rate of the deactivation is strongly dependent on the operating conditions. In the liquid-phase hydrogenation, both the catalytic activity and the ethylene selectivity declined during the first 60 h of operation and then stabilized. This was not observed in the gas phase. Here, the activity and the selectivity passed through a maximum. Temperature programmed analyses of spent catalysts from liquid and gas phase processes revealed that the amount and nature of the species adsorbed on the catalyst surfaces are different for these two types of processes. In the case of a gas phase process, the amount of oligomers formed on the catalyst surface was much higher. In the liquid-phase hydrogenation, these compounds showed a lower H/C ratio and were adsorbed more strongly in comparison to those from the gas phase process.