The interaction of CO with precrystalline Rh-4 and Rh-13 structures is analyzed by means of intermediate neglect of differential overlap (INDO) calculations. The strength of the interaction, as well as the relative contribution of the sigma and pi bonds to the stabilization of CO on the different sites, is studied as a function of the electronic characteristics of the cluster surface. Larger binding energies (BE) are found on the electron-deficient structures and are associated with larger sigma interactions. This brings about a greater charge transfer to the metal moiety and a subsequent geometric distortion due to the weakness of the Rh-Rh bonds in comparison to the Rh-C bonds that are formed. The larger activation energy for diffusion between the sites defined on the smaller structures which has been experimentally derived from C-13 NMR determinations is associated with the large BE for adsorption on electron-deficient sites. These sites are present in higher concentration in the smaller structures.