A detailed study of nitric acid formation in gas-phase ionic aggregates of ammonium nitrate is reported. Fast-atom bombardment was used to sputter cationic ammonium nitrate complexes from neat ammonium nitrate crystals. The sputtered complexes are of the form (NH4NO3)(n)NH4+ and have been detected in the range of n = 1-43. The formation of nitric acid from gas-phase cationic complexes of ammonium nitrate is shown to occur by a rearrangement of NH4+(NO3-)NH4+, the core ion of the first solvent shell. Metastable and collisional-activation dissociation spectra show that a completed first solvent shell exists when the core ion is solvated by the hydrogen bonding of six NH4NO3 molecules to the six available hydrogen atoms of the two ammonium ions, yielding (NH4NO3)(7)NH4+. The metastable loss of HNO3 occurs only from complexes with less than six ligands; a fully solvated core ion cannot rearrange. Collisional activation of larger complexes (n > 7), under multiple-collision conditions, induces sequential losses of NH4NO3 until an ion with n < 7 (incomplete first solvent shell) is formed, at which point rearrangement may occur and one nitric acid molecule may be lost.