In situ high-energy surface X-ray diffraction was employed to determine the surface structure dynamics of a Pd(100) single crystal surface acting as a model catalyst to promote CO oxidation. The measurements were performed under semirealistic conditions, i.e., 100 mbar total gas pressure and 600 K sample temperature. The surface structure was studied in detail both in a steady gas flow and in a gradually changing gas composition with a time resolution of 0.5 s. The experimental technique allows for rapid reciprocal space mapping providing the complete information on structural changes of a surface with unprecedented time resolution in harsh conditions. Our results show that the (√5 × √5)R27°-PdO(101) surface oxide forms in a close to stoichiometric O2 and CO gas mixture as the mass spectrometry indicates a transition to a highly active state with the reaction rate limited by the CO mass transfer to the Pd(100) surface. Using a low excess of O2 in the gas stoichiometry, islands of bulk oxide grow epitaxially in the same (101) crystallographic orientation of the bulk PdO unit cell according to a Stranski-Krastanov type of growth. The morphology of the islands is analyzed quantitatively. Upon further increase of the O2 partial pressure a polycrystalline Pd oxide forms on the surface.