The hydrogenation of 1,4-pentadiene, E-1,3-pentadiene, and a Z/E-1,3-pentadiene mix have been studied over a Pd/alumina catalyst in the liquid phase at 313 K. 1,4-Pentadiene was found to react in a manner similar to 1-pentene. However, E-1,3-pentadiene caused rapid deactivation of the catalyst. When a Z/E-1,3-pentadiene mixture was hydrogenated no such deactivation was observed and the initial rate of E-1,3-pentadiene hydrogenation was reduced. It is proposed that the adsorbed Z-1,3-pentadiene disrupts pi-stacking of the E-isomer. The pentene isomer selectivity indicated that 3,4-addition was favoured, which indicated that the internal double bond was more reactive than the terminal double bond. The reaction of 1,4-pentadiene in competition with 2-pentyne resulted in an increased rate of reaction for both reactants. Whereas the reaction of Z/E-1,3-pentadiene in competition with 2-pentyne or 1-pentyne resulted in a decreased rate of reaction for both reactants. In this case the alkene isomer selectivity could be modelled by simple averaging of the individual selectivities. (C) 2007 Elsevier B.V. All rights reserved.