The selective semi-hydrogenation of isomeric pentynes was studied in the gas phase in a static circulation system over PdZr and PdCuZr alloys activated by HF dissolution. The catalysts were characterized by X-ray photoelectron spectroscopy and X-ray probe microanalysis. 2-Pentyne was found to be more reactive over PdZr, which was attributed to strong self-poisoning by I-pentyne. The two reactants, in turn, exhibited similar reactivities over PdCuZr. The reactivity, however, changed drastically in competitive hydrogenations: the compounds reacted more slowly and 1-pentyne was always more reactive. The reactivity difference was particularly pronounced over PdCuZr. These observations could be interpreted by competitive adsorption of the reactants. Attempts to reactivate the self-poisoned (aged) PdZr sample used in the hydrogenation of I-pentyne by applying various treatments (evacuation and treatment in hydrogen at different temperatures) were unsuccessful. This is due to the accumulation of carbonaceous deposits, Pd dissolution, and crystallization of the amorphous matrix.