The adsorption properties of CO molecules adsorbed on free and MgO supported Au clusters have been investigated by means of a first principles density functional theory (DFT) approach. We have focused our attention on the vibrational properties of CO adsorbed on gas-phase Au3−7, Au12, and Au22 cluster anions, and on Au3−5, Au12, and Au22 clusters deposited on MgO/Ag(001) films or on the bare MgO(001) surface. We also considered a full gold monolayer deposited on MgO/Ag(001). CO does not bind to the high-coordinated Au atoms of one- or two-dimensional (1D or 2D) gold clusters or islands but only to the low-coordinated atoms at the periphery of these structures. A red-shift of about 50−60 cm−1 is found in ω(CO) for gold clusters deposited on MgO/Ag(001) thin films compared to the same clusters deposited on bare MgO(001) or to the neutral gas-phase counterparts. This shift, due to the occurrence of a charge transfer from the metal support to the deposited gold cluster, is found only for small 1D or 2D cluste...