The UV photochem. of mol. adsorbed O on Pd(111) was studied using pulsed laser light with 6.4 eV photo energy. Three processes occur upon irradn.: desorption of O, conversion between adsorption states, and dissocn. to form adsorbed at. O. By using time-of-flight spectroscopy to detect the desorbing mol. O and post-irradn. thermal desorption spectroscopy (TDS) to characterize the adsorbate state, a detailed picture of the photochem. processes is obtained. The O mol. desorb with low translational energies from the satd. surface. The conversion of adsorbed mol. between binding states is induced by the photoinduced build-up of at. O on the surface. Anal. of a proposed reaction model reproduces the obsd. data and yields detailed rates. Polarization anal. indicates that the photochem. processes are initiated by electronic excitations of the substrate.