Particle size affects the activity and selectivity to partial hydrogenation of 1-pentyne over oxide-supported palladium nanoparticles. Larger particles are intrinsically more selective because of the weaker bond strength of 1-pentene. In situ X-ray absorption near edge structure (XANES) at the Pd L(3) edge revealed the formation of a carbide-like phase as soon as the catalyst is exposed to alkyne, irrespective of particle size. The newly formed phase prevented hydride formation. Surface poisoning of the palladium carbide by alkyne is responsible for the constantly high selectivity, up to almost complete conversion. At almost 100% conversion, all catalysts show low selectivity. The lack of significant pentyne adsorption on the surface causes pentene to undergo consecutive reactions, such as isomerization and complete hydrogenation. The structure of the catalyst was that of carbide-like phase and did not change. Palladium hydride did not form under any of the conditions. Exposure of a carbided catalyst to pure hydrogen leads to partial reversal of the structure. Hydride is not essential for complete hydrogenation to occur. (C) 2011 Elsevier Inc. All rights reserved.