We have performed the first "high-pressure" X-ray photoelectron spectroscopy (XPS) study on the palladium, hydrogen, and olefin (trans-2-pentene) system to gain better insight into the hydrogenation reaction. We report here data collected with the use of a Pd(111) single crystal and a polycrystalline foil. Hydrogenation was observed on polycrystalline foil (RT and 373 K) but not on Pd(111) single crystal, as revealed by on-line mass spectrometry. We observed the reaction in the presence of a huge amount of carbon (up to 73%) in the information depth of XPS. Mainly graphite was present on Pd(111), whereas other components, C-H and C-Pd, were also formed on the foil to a much greater extent. C-Pd characterizes a carbon species in the interaction with palladium, whereas C H represents hydrogenated carbon, including chemisorbed species. The d-band of the foil showed a remarkable upshift toward E-FERMI compared with Pd(111). We concluded that the differences found in the valence and the C1s region are indicators of different electronic structures that contribute to the variation in activity. The palladium foil lost its activity at an elevated temperature (523 K), most probably because of desorption of hydrogen. From additional UPS measurements, we concluded that trans-2-pentene is hydrogenated in sigma-bonded chemisorption modus, at least in UHV conditions. (C) 2004 Elsevier Inc. All rights reserved.