Raman spectra of solid nitric acid hydrates (NAM, alpha- and beta-NAD, alpha- and beta-NAT, and NAP) are obtained in the low-frequency region 20-175 cm(-1) where phonon bands show characteristic patterns. This fingerprint information, intimately related to the structure and symmetry of the unit cell, is well suited for observation of phase changes in solid nitric acid hydrates and allows the distinction of mixtures of different hydrate phases. The low-frequency spectra are correlated with the spectra of the respective symmetric NO stretching vibration (1000-1080 cm(-1)), with literature data, and with X-ray diffraction patterns.