Vibrational spectroscopy, X-ray diffraction, and electron microscopy have been applied to investigate the crystallization of concentrated amorphous nitric acid (0.5 ≤ x ≤ 0.95). Crystalline nitric acid monohydrate (HNO3 · H2O) is the major hydrate phase detected together with traces of metastable nitric acid dihydrate (HNO3 · 2H2O) and pure crystalline nitric acid. Nitric acid tetratohydrate (4HNO3 · H2O) was not achievable. The diffraction data show nitric acid monohydrate as a highly symmetrical structure, which bestows clearly laid out infrared, Raman and inelastic neutron scattering spectra. The low frequency region is still a matter of discussion since experiment and theory are not in accordance. The microscopic picture underlines the model character of nitric acid monohydrate, since the crystalline particles adopt a spherical shape, on various agglomeration stages, which minimizes the interference of scattering on the spectroscopic data.