A series of yttrium oxide-carbonyls are prepared via a laser vaporization supersonic cluster source in the gas phase and identified by mass-selected infrared photodissociation (IRPD) spectroscopy in the C-O stretching region and by comparing the observed IR spectra with those from quantum chemical calculations. For YO(CO)(4)(+), all four CO ligands prefer to occupy the equatorial site of the YO unit, leading to a quadrangular pyramid with C-4 nu symmetry. Two energetically nearly degenerate isomers are responsible for YO(CO)(5)(+), in which the fifth CO ligand is either inserted into the equatorial plane of YO(CO)(4)(+) or coordinated opposite the oxygen on the C-4 axis. YO(CO)(6)(+) has a pentagonal bipyramidal structure with C-5 nu, symmetry, which includes five equatorial CO ligands and one axial CO ligand. The present IRPD spectroscopic and theoretical study of YO(CO)(n)(+) extends the first shell coordination number of CO ligands in metal monoxide carbonyls to six. The transition from equatorial to axial CO chemisorption in these yttrium oxide-carbonyls is fortunately observed at n = 5, providing new insight into ligand interactions and coordination for the transition metal oxides.