Many important oxidative reactions, such as CO oxidation, take place on metal surfaces at high temperatures and partial pressures. Understanding the atomic processes involved in these catalyzed reactions are of great importance and may be achievable by observations of the adsorbate-induced surface structure under temperatures and pressures relevant to working catalysts. Many of the prior studies, however, have only considered quenched-in structures with no dynamic interaction between the metal surface and the gas phase. This presentation describes in-situ synchrotron x-ray studies of the Cu (001) surface as a function of pO2, the oxygen partial pressure, and temperature. We utilize a controlled-flow reaction chamber specially constructed to mount onto an eight-circle diffractometer at the Advanced Photon Source. The chamber allows the flow of oxygen, hydrogen, and argon mixtures with pO2 ranging from 760 to 1×10-12 Torr and sample temperatures variable from 25 to 1000 °C. After reaching a critical pO2, oxygen adsorbs onto the initially clean Cu (001) surface, resulting in the rapid nucleation and growth of c(2×2)-O domains. Domain formation is concurrent with a small in-plane surface contraction and a large out-of-plane surface expansion associated with a compressive adsorbate-induced surface stress. The often reported (2√2×√2)R45 reconstruction is observed only below ~ 150 °C. Relationships between the different surface structures, subsurface oxygen, surface stress, and surface reactivity will be discussed.