The adsorption of CO on Pt(lll) was studied by picosecond infrared-visible sum frequency generation (SFG) vibrational spectroscopy in a pressure range from 10(-7) to 500 mbar and in a temperature range of 160-400 K. At low pressure the experiments were complemented by TPD, LEED, and AES. Terminally bonded (on-top) CO was the only species observed between 160 and 400 K, independent of gas pressure. The CO stretching frequency was blue-shifted by about 15 cm(-1) with increasing pressure (up to 2097 cm(-1)), but no evidence for high-pressure CO species or surface roughening was found. The influence of defects was also investigated. CO adsorption on a defective (nonannealed) Pt(lll) surface yielded peaks that were slightly broadened but otherwise identical to the defect-free surface. At 160 K, a second peak at 2085 cm(-1) evolved above 50 mbar of CO. TPD revealed that under these conditions residual (contaminant) water adsorbs on the surface. The coadsorption of water and CO red-shifted the terminal CO peak by about 15 cm(-1), resulting from the substrate-mediated interaction of CO and water.