Abstract We report measurements of the temporal oscillatory oxidation rates of CO over a polycrystalline Pt wire. The experiments were conducted near atmospheric pressure in a clean flow reactor system. Reproducible oscillations in both the temperature of the Pt wire and in the rate of CO 2 production were found over a wide range of gas compositions, 0.001 < P CO P O 2 < 0.045, and temperatures, 150° C < T g < 350° C, where PCO and po 2 are the respective partial pressures of carbon monoxide and oxygen in the gas stream, and T g is the temperature of the gas. The oscillations are believed to occur between two branches of a Langmuir-Hinshelwood reaction mechanism. It is suggested that the slow formation and removal of subsurface oxygen drives the reaction between the two branches. A simple kinetic model based on this hypothesis gives excellent qualitative agreement with the observed oscillations.