Pd (100) with mesoscopic defects was prepared, and oxygen adsorption on this surface was studied by photoemission electron microscopy (PEEM). The defect surfaces behaved differently towards oxygen adsorption from the Pd (100) surface. Besides the oxygen species adsorbed on the surface, the formation of surface oxide and subsurface oxygen on different facets, as well as surface restructure, was observed by PEEM during oxygen adsorption.