The O-3 dissociation and CO adsorption on various gold single crystal surfaces were investigated using X-ray photoelectron spectroscopy, temperature-programmed desorption, and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS). Dissociation of O-3 proceeded on the Au(111) surface at 323 K, whereas no O-3 dissociation occurs on the Au(100) surface, indicating that the O-3 dissociation on gold strongly depends on the structure of the gold surface. PM-IRA peaks of CO adsorbed on Au(111) and Au(100) were observed around 2,080 cm(-1) at CO pressures above 0.5 Torr at 273 K. On the other hand, the peak of CO adsorbed on step sites was observed at 2,117 cm(-1) above 0.01 Torr for Au(311). Thus, CO adsorbed preferentially onto the step sites of the gold surface.