The IR Spectra of carbon monoxide adsorbed on different forms of platinum, iridium and copper (flat and rough surfaces) have been investigated. The results of IR transmission experiments on the silica supported metals are presented and compared with the data from IRAS (IR reflection adsorption spectroscopy) experiments on the unsupported metals. It is shown that the coverage dependency of the IR band frequency Δν CO (θ) is independent of the state of the metal if the formation of islands of CO is taken into consideration. This was proven by isotopic dilution experiments with 12 CO/ 13 CO mixtures. It is suggested that many apparent discrepancies and irreproducibilities in the literature can be explained by an uncontrolled formation of islands. At a given surface coverage. Group VIII metals reveal a lower ν co value when CO is adsorbed on rough surfaces (or small particles) than on smooth surfaces (or large particles). The reverse is true for the lb metal Cu. The differences between supported and unsupported metals can be ascribed to differences in the electronic and geometric structure of the surface. The differences between adsorption of CO on Group VIII and lb metals are discussed in terms of a difference in bonding of CO on these two groups of metals.