Iridium and rhodium are group IX elements that can both catalytically reduce NO. To understand the difference in their reactivity toward NO, the adsorption forms of NO onto clusters of Ir and Rh are compared using vibrational spectra, recorded via infrared multiple-photon dissociation spectroscopy. The spectra give evidence for the existence of at least two specific adsorption forms. The main Ir6+NO isomer is one in which NO is dissociated, whereas one other is a local minimum structure in the reaction pathway leading to dissociative adsorption. In contrast to adsorption onto Rh6+, where less than 10% of the isomeric population was found in the global minimum associated with dissociative adsorption, a substantial fraction (about 50%) of NO dissociates on Ir6+. This higher efficiency is attributed to a considerably reduced activation barrier for dissociation on Ir6+. The key chemical property identified for dissociation efficiency is the cluster's affinity to atomic oxygen.