The X(2)A(2)(1) band of the photoelectron spectrum of NO3- is theoretically calculated in the framework of a multimode vibronic coupling model. Linear coupling between and within the X 2A; and the B(2)E’ electronic states of NO3 is found to be the important mechanism governing the dynamics. The necessary coupling constants are obtained from the ionization potentials of NO3- calculated at different geometries using ab initio Green’s functions. Comparison of the theoretical with experimental results [A. Weaver, D.W. Arnold, S.E. Bradforth, and D.M. Neumark, J. Chem. Phys. 94(3), 1740 (1991)] for the PE spectrum shows good agreement if a temperature of the NO3- anion of 455 K in the experiment is assumed. A general theoretical treatment of thermal effects in vibronically coupled electronic states is presented. The ground electronic state potential surface of NO3 is discussed in the framework of the vibronic coupling model. A shallow minimum at a C-2v geometry with a barrier height of 0.006 eV relative to the minimum energy D-3h configurtion is found. It is too weak to deform the effective geometry of the molecule from D-3h to C-2v. Mode-mode coupling is found to play a relevant role in the ground electronic state of NO3, in general.