Vibrational spectra of Rh 7 O m + (m = 4−7, 12, 14) were measured in the 300−1300 cm −1 range via infrared multiple photon dissociation (IRMPD) spectroscopy. For the oxygen poor cluster sizes, Rh 7 O m + (m = 4−7), IRMPD spectra were recorded through photodissociation of Rh 7 O m + −Ar complexes. IR spectra for Rh 7 O m + (m = 12, 14) were recorded via the release of an O 2 molecule from Rh 7 O m + producing Rh 7 O m−2 + ; no O 2 loss was observed from Rh 7 O m + (m = 4−7, 8, 10). By comparison with the calculated vibrational spectra of several stable isomers obtained using density functional theory, these IR spectra are assigned to geometrical structures. For m = 4−7, all O atoms are bound to Rh atoms only, with a transition of the Rh core from capped octahedral to capped prismatic between m = 5 and m = 6. For the oxygen-rich Rh 7 O 12 + and Rh 7 O 14 + , molecular O 2 is adsorbed on a bridge site between two Rh atoms. The frequencies of the bands observed bands signal that the O 2 molecules are activated, indicating that rhodium oxide clusters with Rh 2 O 3 composition are still capable to donate electrons to activate O−O bonds in adsorbed O 2 .