Decomposition reactions of NO molecules on gas-phase Rh_n⁺ (n = 6–9) clusters were investigated by gas-phase thermal desorption spectrometry and density functional theory calculations. We found that NO adsorbs on the clusters, forming Rh_nN_xO_x⁺ at room temperature. Upon heating, NO desorption was observed below 800 K. Above 800 K, while for n = 7 and 8, each of Rh₇N₃O₃⁺, Rh₇N₄O₄⁺, and Rh₈N₃O₃⁺ was found to release an N₂ molecule, no N₂ formation was clearly observed for Rh_6,9N_xO_y⁺. We considered that both Rh₇N₃O₃⁺ and Rh₈N₃O₃⁺ have at least two dissociated NO molecules, while Rh₆N_xO_x⁺ (x = 1–3) has one or less. Our computational results for Rh₈N₃O₃⁺ suggested that the formation of an N–N bond in the Rh₈N₃O₃⁺ structure must overcome an energy barrier of ∼2 eV, which is the highest among the suggested possible reaction pathways. These findings suggested that the size-dependent activity of NO decomposition is governed primarily by how NO molecules are adsorbed on Rh_n⁺ clusters, i.e. whether two or more N atoms from dissociated NO molecules exist in the NO adsorbed clusters, and secondly, by the readiness of the N–N bond formation.