The adsorption of CO on Pd(I 11) and on Al2O3-supported Id nanoparticles was studied by picosecond infrared-visible sum frequency generation (SFG) vibrational spectroscopy in a pressure range from 10(-7) to 1000 mbar and in a temperature range of 100-520 K. Under ultrahigh vacuum (UHV), the samples were further characterized by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and temperature-programmed desorption (TPD). Identical high coverage (saturation) CO structures were observed on Pd(l 11) under UHV conditions (10(-7) mbar, 100 K) and at high pressure (e.g., I mbar, 190 K). No indications of pressure-induced surface rearrangements of Pd(l 11) were evident from SFG and LEED. SFG spectra of CO adsorption on "defect-rich" Pd(l 11) revealed an additional peak that was attributed to adsorption on defect (step or edge) sites. The CO adsorbate structure on supported Pd nanoparticles was found to be different from that on Pd(I 11) and to be more similar to that on stepped or strongly sputtered Pd( I I I), At low pressure, the adsorption site occupancy depended on the particle surface structure and temperature. CO preferentially adsorbed in bridge sites on well-faceted Pd particles, while on more defective Pd particles, on-top sites were occupied as well. However, at 200 mbar CO, an adsorption site occupancy was obtained that was nearly independent of the particle surface structure. While the surface structure of the Pd particles remained unchanged upon high-pressure gas exposure, an annealing treatment to 300-400 K was able to order the Pd particle surface. Gas mixtures of CO and hydrogen on Pd(l 11) showed SFG spectra similar to the pure CO case indicating the absence of a strong interaction between CO and hydrogen.