The dissociation of CO on size-distributed Rh particles supported on a thin alumina film has been studied with high resolution X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). Adsorbed CO dissociates upon heating to temperatures above 300 K. The dissociation activity is dependent on the island size, exhibiting a maximum for islands with around 1000 atoms. We have identified size-dependent changes in the C Is photoelectron spectra for these CO-Rh systems occurring at temperatures lower than the onset of both the dissociation and desorption processes. These changes are interpreted as being due to adsorbed CO shifting into more highly coordinated sites. The dissociation activity is directly correlated to the availability of these sites, where the observed dissociation is proposed to occur. These results can be interpreted primarily in terms of the size and shape of the deposited Rh particles.