The hydrogenation of a number of C(5) olefins (pent-1-ene, trans-pent-2-ene, cis-pent-2-ene, trans-1,3-pentadiene and a technical mixture of 1,3-pentadiene) over a 1% Pd/Al(2)O(3) catalyst has been studied using in situ infrared spectroscopic methods to observe the changes in the gas phase molecules during the course of the reaction. Whereas trans-pent-2-ene is directly hydrogenated to pentane, the reaction profile for cis-pent-2-ene indicates a consecutive process involving the formation of gaseous trans-pent-2-ene as a reaction intermediate. Extending these studies to trans-1,3-pentadiene shows the terminal double bond to be hydrogenated first to produce trans-pent-2-ene in the gas phase, which is then subsequently hydrogenated to the alkane. A reaction scheme is proposed that defines how the molecules are partitioned between the gaseous and adsorbed phases. This scheme makes use of a previously postulated two-site adsorption model. Analysis of a technical grade of 1,3-pentadiene indicates the trans-monoene to play a significant role in the stepwise hydrogenation process. (C) 2009 Elsevier B.V. All rights reserved.