The mass-selected dirhodium oxide Rh2O9+ was prepared from the reaction of laser ablation of rhodium atoms with O2/He mixtures. Infrared spectra of Rh2O9+ are measured by infrared photodissociation spectroscopy in the rhodium-oxygen and OO stretching frequency region observed at 1254, 876 and 1536 cm−1. To assign the experimental spectra, total 43 tautomeric isomers with doublet, quartet, sextet and octet states are optimized and analyzed to compare the experimental with simulated spectra derived from density functional calculations. Further CCSD(T) single point energy confirmed the most stable isomer was the dinuclear sextet Rh2O9+ cation with a pseudo five-numbered ring as well as one Rh-O-Rh core terminally bonded to one superoxide, two terminal Rh-O, and two dioxygen groups. The three observed bands at 876, 1254 and 1536 cm−1 bands are exactly assigned to the terminal Rh-O, superoxide OO and weak dioxygen OO stretching modes compared to calculated vibration frequencies of the sextet Rh2O9+ cation(isolmer 29). Bonding analysis indicate that both rhodium of the Rh2O9+ cation observed display + 5 and + 3 formal oxidation state, respectively.