The authors performed extensive HREELS studies on the O₂/Pd (111) chemisorption system to identify and characterize neg. ion resonances in inelastic electron scattering by mol. O adsorbed on Pd. Such resonances may play an important role in the photochem. of this system. The O₂/Pd (111) system is of particular interest because 3 different chemisorbed states are known to exist and its photochem. is well characterized. The authors found a pronounced resonance feature at .apprx. 1 eV. The dependence of the scattered electron intensity is discussed as a function of both primary electron energy and final scattering angle in terms of the structure of this resonance.