We investigated free and water-solvated gas phase nitrate anions, NO3−(H2O)n (n = 0–6), by photodetachment photoelectron spectroscopy and theoretical calculations. We obtained the electronic structure and electron binding energies of the free and solvated NO3− at three detachment photon energies, 4.661, 6.424, and 7.866 eV. The ground and two low-lying electronic excited states of the NO3 radical (X 2A2′,A 2E″,B 2E′) were observed at the 6.424 and 7.866 eV photon energies. The photoelectron spectra of the solvated nitrate complexes are similar to that of the bare NO3−, except that they become broadened and diffuse due to the solvation. The spectrum of NO3−(H3O)3 showed a resolved vibrational progression of the N–O symmetric stretching (1000 cm−1), suggesting the cluster possesses a high symmetry. NO3− and NO3 were calculated at various levels of theory. Based on the good agreement between density functional theory calculations and experiment for NO3− and NO3, we carried out systematic calculations for NO3−(H2O)n (n = 1–6) using primarily density function theory methods. The calculations indicate that NO3−(H2O)n (n = 1–3) are all planar, with the first three H2O forming the first solvation shell around NO3−, giving rise to a highly symmetric C3h NO3−(H2O)3. The next three waters form a second solvation shell without direct contact with NO3−. The C3h NO3−(H2O)3 solvation structure was observed to be rather robust and largely preserved in the larger clusters. © 2002 American Institute of Physics.