In the present contribution CO adsorption on palladium aggregates was examined with IR-visible sum frequency generation vibrational spectroscopy. Supported Pd nanoparticles of different mean size and surface roughness were prepared by electron beam evaporation on an ordered Al 2 O 3 thin film formed on NiAl(110). SFG spectra were acquired for a wide range of CO pressures (from 1 Γ 10 --7 to 200 mbar) at 190 to 300 K. At low pressure, evidence for a particle structure dependence of the CO adsorption geometry was found, with on-top CO sites significantly occupied on defective particles. On the other hand, under a high pressure regime, discrepancies between rough and smooth well-faceted particles were mostly reduced, with the relative fraction of on-top CO nearly equal for both cases. As a reference model, adsorption on well-defined Pd(111) single crystals was monitored as a function of CO pressure up to 1 bar and contrasted to the work done on aggregates. Under ultrahigh vacuum the SFG spectra on large Pd particles closely resembled the Pd(111) response. However, for pressures higher than 1 mbar at 190 K twofold bridge bonded sites were no longer populated on the Pd(111) surface while the bridge bonded occupancy was significant on Pd aggregates for all pressures studied.