The vibrational modes of the tilting motion initially decrease for CO adsorbed on Co(0001) in the ( √ 3 × √ 3)R30 • structure at 1/3 ML coverage. At higher coverages, the vibrational modes follow the trend from Pt(111); the modes increase as the motion becomes restricted by the coadsorbates. Pendulum and desorbing modes remain relatively constant until a coverage of 5/9 ML, but then also increase with coverage. The change in the C-O stretching frequency can be correlated with the adsorption site. Up to 1/3 ML, CO is exclusively adsorbed on atop sites and the stretching frequency remains constant. At 4/9 ML CO, moves to sites with higher-coordination numbers and the stretching mode decreases by 100 wave numbers. The occupation of bridge and 3-fold hollow sites leads to a decrease of the stretching mode, which is also confirmed by the results on Pt(111).