Abstract. The structure of supported platinum catalyst (2 wt% Pt/Al₂O₃) has been determined during the sudden change in the activity of the catalyst, referred as "ignition", during oxidation of carbon monoxide. The catalyst was prepared by incipient wetness impregnation method. The active phase of the catalyst is shown to be a highly disordered platinum oxide. This surface oxide increases with the increase of carbon monoxide conversion during the ignition process, which further increased the conversion, making the process autocatalytic. The QEXAFS spectra collected at Pt L₃ edge with a time resolution of 0.5 sec are compared with the standard EXAFS that are generally collected in transmission mode and are shown to be useful to extract valuable information about the changing structure of the catalyst during the ignition.