spatially resolved techniques, such as photoemission electron microscopy (PEEM) and ellipsomicroscopy for surface imaging (EMSI), have been particularly useful in the investigation of pattern formation during CO oxidation on platinum catalysts. One surprising result of these studies has been the discovery of subsurface oxygen on Pt(100). The formation of subsurface oxygen has been reported previously on Pt(100) during CO oxidation at low pressures (< 1 x 10(4) Torr). This communication reports the formation of Subsurface oxygen at intermediate pressures (similar to 0.1 Torr). These observations show that subsurface oxygen plays a role in cataltic CO oxidation and pattern formation, which has implications for catalysts operated at higher pressures. New microkinetic models of CO oxidation that incorporate subsurface oxygen are discussed. Results of these models are qualitatively similar to experimental observations of subsurface oxygen, confirming the importance of this species in the reaction dynamics. (c) 2005 Elsevier B.V. All rights reserved.