The effect of arsenic poisoning of nickel catalysts for ethene adsorption was studied by the magnetic method and by infrared spectroscopy. Ethene adsorbed dissociatively on clean nickel. Arsine preadsorbed on the surface of nickel inhibited ethene dissociation and so reduced the ethene bond number at low coverage from 8 for clean nickel to 1.3 for nickel preadsorbed with arsine to the amount of 0.55 mmol/g nickel. Heating the arsine-covered nickel to 672-773 K led to segregation of arsenic into the bulk of the nickel, lessening its inhibiting effect for ethene dissociation. Infrared spectroscopy for ethene adsorbed on nickel-arsenic bimetallic catalysts showed that dissociation and self-hydrogenation of adsorbed ethene on nickel was inhibited by arsenic. For the bimetallic catalyst with As:Ni mole ratio of 0.16:1, only pi-bonded ethene was observed by infrared spectroscopy.