The extended Huckel method has been applied to nitrogen dioxide for a series of bond angles extending from 90 to 180" and for three values (1.50, 1.75, and 2.00) of the parameter K appearing in the Wolfsberg-Helmholtz formula. For K = 2.00, very good agreement with experiment is obtained for the bond angle of NO2-, NO2, and NO2+, while the experimental bending force constants are reproduced in a satisfactory fashion. Comparison is made between Walsh's diagram for AB2 molecules and the curves obtained for the orbital energies. The adiabatic ionization potential of NO2 is found equal to 10.98 ev, which coincides with a recent experimental determination by photoelectron spectroscopy.2 The first excited state of NO2+ is predicted to be bent. It is found that upon dimerization all the occupied b2 orbitals of NO2 give rise to some destabilization; this is suggested to be the reason for the low dissociation energy of the dimer. The loss of color associated with the dimerization appears to be linked with the stabilization incurred by the 4al orbital of NO2.