The chemisorption of CO on an Fe (110) surface has been investigated at 120 K using high resolution electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). At CO exposures up to 0.7 L two vibrational losses around 456 cm−1 and 1890 cm−1 are observed both shifting to higher frequencies with coverage. These losses can be attributed to the symmetric vibrations of ’’on top’’ bonded CO molecules. The intensity of both losses increases continuously in this exposure range. Above 0.7 L the low frequency loss continues to grow whereas the intensity of the high frequency loss remains constant up to saturation coverage. The latter observation may be explained by depolarization of the dynamic dipole moments of the CO molecules. Simultaneously at 0.7 L a new loss appears around 360 cm−1 which may be attributed to the frustrated rotation of the adsorbed CO molecule when changing the adsorption site symmetry from C2v to Cs. Adsorption models are presented which are in agreement with both the LEED and EELS observations.