This article focuses on the microscopic origin of oscillations in the rate of CO oxidation at atmospheric pressures on Pt group metals. We start by briefly reviewing several existing models for oscillatory CO oxidation on such Surfaces at pressures ranging from ultrahigh vacuum to several bar. We describe our observations for this reaction on Pt(110) and Pd(100) at atmospheric pressures obtained with a scanning tunnelling microscope inside a microflow reactor. The combination of STM movies and the simultaneously determined partial pressures and reaction rate reveals that by switching between a CO-rich flow and an O-2-rich flow. and vice versa, we can reversibly oxidize and reduce the Pt and Pd surfaces. We find that on both surfaces the formation of the oxide, seen in the STM images, is accompanied by a stepwise increase in the reaction rate and a change in the reaction mechanism. The switching between the reduced metal and the surface oxide exhibits significant hysteresis as a function of CO pressure and for both Pt and Pd there exists a CO pressure window in which both the metal and tile oxide state are stable. This bistabililty leads to spontaneous reaction oscillations on Pd( 1 0 0). Under Our atmospheric conditions thew oscillations are not due to the 'familiar' bistability in Langmuir-Hinshelwood kinetics, in contrast with common 'wisdom'. Instead, tile high-pressure reaction oscillations reflect the periodic switching between the low-reactivity metallic surface and the high-reactivity oxide surface, Our results show further that effects that are held responsible for low-pressure reaction oscillations are not relevant at atmospheric pressure. (c) 2005 Elsevier B.V All rights reserved.