We report a synchrotron radiation vacuum ultraviolet photoionization study of the hydroperoxyl radical (HO2), a key reaction intermediatein combustion and atmospheric chemistry as well as astrochemistry, using double imaging photoelectron photoion coincidence spectroscopy.The HO2radical is formed in a microwave discharge flow tube reactor through a set of reactions initiated by F atoms in a CH4/O2/He gasmixture. The high-resolution threshold photoelectron spectrum of HO2in the 11 eV–12 eV energy range is acquired without interferencesfrom other species and assigned with the aid of theoretically calculated adiabatic ionization energies (AIEs) and Franck–Condon factors. Thethree vibrational modes of the radical cation HO2+, the H–O stretch, the H–O–O bend, and the O–O stretch, have been identified, and theirindividual frequencies are measured. In addition, the AIEs of the X3A′′ground state and the a1A′first excited electronic state of HO2+areexperimentally determined at 11.359±0.003 eV and 11.639±0.005 eV, respectively, in agreement with high-level theoretically computedresults. Furthermore, the former AIE value provides validation of thermochemical networks used to extract the enthalpy of formation of theHO2radical.