The partial and total oxidation of ethene was studied on clean and oxidized polycrystalline Pd in a UHV high-pressure cell. The model catalyst was prepared under UHV conditions and characterized by thermal desorption and Auger and Raman spectroscopy. Starting from either a Pd or PdO surface, reaction to CO2 is the predominant pathway, and acetic acid is the main byproduct between 453 and 523 K. The route to partially oxidized products, in particular acetic acid, is more favoured on the PdO surface, while on Pd metal acetic acid is formed at lower temperatures than on PdO. On stoichiometric PdO activation for both partial and total oxidation occurs during an induction period, until under working conditions Pd metal, PdO, 'PdCx' and isolated defects are coexistent on the surface. The presence of an extended metal surface favours the follow-up oxidation of byproducts, while the presence of PdO stabilizes acetic acid far beyond 473 K.