Raman spectra at 244 nm are obtained in situ from several Pt/gamma-Al2O3 catalysts under gas flows containing O-2, CO, NO2, and H, at temperatures from 20 to 600degreesC. With O-2 Or NO2, a broad peak at 571 cm(-1) is observed which we attribute to the Pt-O stretch mode of atomic oxygen. This marks the first direct spectroscopic observation of this fundamental species on a real catalyst. The peak can be easily removed with CO or H-2, even at room temperature, and the process is completely reversible. With NO2 exposure, several peaks associated with surface NO, species also appear, the most prominent of which is a nitrate line at 1048 cm(-1). Under reaction conditions, at 350degreesC with a stoichiometric CO/O-2/N-2 gas stream, the steady-state O coverage on the Pt particles is about half of the maximum coverage achieved with only an O-2 flow. This observation paves the way for future in situ studies of catalytic mechanisms addressing the role of atomic O as an intermediate.