The influences of water poisoning on isoprene-selective hydrogenation over delta-alumina-supported eggshell Pd catalysts were investigated by test reactions at 40 degrees C and 420 psig of total pressure. Both fresh and water-poisoned catalysts were characterized by fast Fourier transform infrared (FFT-IR) spectroscopy and temperature-programmed desorption (TPD) of hydrogen. The catalytic performance tests showed that water poisoning suppresses the hydrogenation of isoprene and the double-bond migration of 2-methyl-1-butene and 3-methyl-1-butene to 2-methyl-2-butene, whereas it slightly facilitates the selectivity to the formation of isopentenes. FFT-IR spectroscopy characterizing CO adsorbed on the fresh and water-poisoned catalysts indicated that the bond strength between CO and palladium was weakened by the adsorption of H2O on Pd, suggesting that the adsorbed water abstracts electrons from Pd clusters. The TPD results further indicated that the formation of beta-hydride was inhibited by this water adsorption. Together with the catalytic performance test, FFT-IR, and TPD results, the alteration of the catalytic properties by water poisoning is associated with the decrease of active sites for hydrogenation, the decrease of the electronic density of the Pd clusters, and the inhibition of beta-hydride formation.