Density functional computations have been performed to determine equilibrium geometrical structures, magnetic properties, and vibrational frequencies of a series of Rh-x(CO)(y) (x less than or equal to 4, y less than or equal to 2) clusters. The results are compared with experiments on CO adsorption on small rhodium. aggregates deposited onto a thin, well-ordered alumina film. The CO stretching frequency as a function of rhodium cluster topology and spin-state is reported and discussed. The considered rhodium monocarbonyls are found to exhibit nonzero magnetic moments. They show a more pronounced dependence of the CO-stretching frequency on the magnetic properties than on the number of rhodium atoms. The CO bonding behavior in the formation of Rh-x(CO)(y) species is rationalized on the basis of the molecular orbital and the natural bond order analysis.