Ab initio molecular orbital methods have been employed to determine the molecular structure, vibrational frequencies, and stability of HOONO. These parameters were determined using quadratic configuration interaction methods with Dunning's correlation consistent basis sets. Low-lying excited states for HOONO have been determined using complete active space self-consistent field (CASSCF) and multireference internally contracted configuration interaction (MRCI) methods. The first excited state (1 (1)A ") is calculated to be 4.19 eV above the ground state of HOONO. Potential energy curves for the ground and excited states are presented.