The UV photochem. of O₂ adsorbed on Pd(111) was investigated by high-resoln. electron energy-loss spectroscopy (HREELS). For excitation with 6.4 eV photons at normal incidence a dissocn. cross section of 4 * 10⁻ⁱ⁹ cm² is found. The dependence of the cross section on polarization and angle of the incident laser light clearly demonstrates that photodissocn. of the peroxo species is initiated by substrate excitations. Symmetry considerations show that direct excitations would give rise to dependences different from those characteristic of a substrate mediated process.