The adsorption of carbon monoxide on rhodium clusters in the size range of 3-15 atoms is studied in the gas phase using the frequency of the internal CO stretch, nu(CO), to probe the bonding situation of the CO. The IR absorption spectra of neutral, cationic, and anionic RhnCO complexes are measured in the frequency range of nu(CO), between 1650 and 2200 cm(-1), using IR multiple photon dissociation spectroscopy. We find that for most clusters adsorption in an atop position (mu(1)) is preferred; however, for some clusters, CO in bridging (mu(2)) or hollow (mu(3)) sites can be identified as well. Comparison with DFT calculations carried out for the smallest cluster complexes RhnCO+/0/-(n = 3 and 4) shows that the experimentally identified CO adsorption sites correspond to the energetically favored positions.