Ab initio calculations have been performed to study the molecular structures and the vibrational levels of the low-lying ionic states ((1)A(1), B-3(2), (3)A(2), (1)A(2), B-1(2), B-1(1), 2 (1)A(1), B-3(1), and (3)A(1)) of nitrogen dioxide. The global regions of the potential energy surfaces have been obtained by multireference single and double excitation configuration interaction calculations. Vibrational calculations using vibrational Hamiltonians have been used for vibrational analysis. The equilibrium molecular structures and a vibrational analysis of these states are presented. The theoretical ionization intensity curves including the vibrational structures of the ionic states are also presented and are compared with the photoelectron spectra. A new assignment of the photoelectron spectra has been proposed.