The first ab initio theoretical study has been performed on the ground state of nitroxide, HNO-, in combination with a comparison study of the ground state of two chemically similar systems, peroxy radical, HO2, and the neutral nitroxyl radical, HNO. An optimum geometry and vibrational frequencies are reported for all species, and results are compared to experimental and theoretical values. A bent geometry is obtained for HNO-, with an equilibrium bond angle of 106.2-degrees, and bond lengths of R(NO) = 1.333 angstrom and R(NH) = 1.045 angstrom. The calculated fundamental vibrational frequencies (in cm-1) are : nu1 (N-H stretch), 3029; nu2 (bend), 1474; and nu3 (N-O stretch), 1183. The bending frequency and the N-O stretching frequency in the ion are similar to the bending frequency and the O-O stretching frequency of HO2, while the N-H stretching frequency of HNO- is similar to the N-H stretching frequency of HNO. In addition, the GAUSSIAN 1 and GAUSSIAN 2 methods are used for the determination of an electron affinity for HNO. The calculated electron affinity of 0.23 +/- 0.10 eV is in reasonable agreement with the experimentally determined value of 0.338 +/- 0.015 eV.