The UV irradn. (hn = 3.5 to 6.4 eV) of NO₂/N₂O₄ adsorbed on top of a NO-covered Pd(111) surface leads to photodissocn. State-resolved detection of the NO mols. released into the gas phase and detn. of the yields as function of angle and polarization provide clear evidence that the dominant primary process consists in electronic excitation of the metallic substrate rather than in direct absorption by the adsorbate.