Gas-phase rhodium oxide clusters, RhnOm+, were investigated by measuring the rate constants of oxidation and thermal desorption spectrometry. RhnOm+ was suggested to be categorized into different states as m/n <= 1, 1 < m/n < 1.5, and 1.5 < m/n in terms of energy and kinetics. For m/n <= 1, the O atoms readily adsorbed on the cluster with a large binding energy until RhO was formed. Under the O-2-rich environment, oxidation proceeded until Rh2O3 was formed with a moderate binding energy. In addition, O-2 molecules attached weakly to the cluster, and Rh2O3 formed RhnOm+ (1.5 < m/n). The energetics and geometries of Rh6Om+ (m = 6-12) were obtained using density functional theory calculations and were found to be consistent with the experimental results.