Raman spectra of mercury(II) nitrate in concentrated aqueous solutions (0.7-7.0M) indicate that the nitrate groups are present as associated ions or covalently bound groups. A gradual alteration in the appearance of the nitrate spectrum with increasing concentration is explained in terms of changes in the nature of mercury-nitrate interaction. A Raman line assignable to the stretching of a mercury-nitrate covalent bond has been observed at c. 270 cm-1. The spectrum of solid Hg(NO3)2,H2O differs from spectra of the aqueous solutions, mainly in the region 1300-1450 cm-1. Additional features are attributed to the influence of hydrogen bonding in the crystal lattice or to correlation field effects (anion-anion coupling).