Infrared spectra of NO2+(H2O)(n) clusters formed in a pulsed discharge source were recorded by vibrational predissociation spectroscopy as a function of cluster size. A significant change in the vibrational spectra and the predissociation product yields were observed at n = 4, suggesting the onset of a rearrangement at this critical cluster size. For NO2+(H2O)(n) with n less than or equal to 3, the clusters possessed red-shifted H2O bands and dissociated primarily by loss of H2O. These were clusters of H2O Ligands bound to a nitronium ion core. At n = 4, we found evidence for the occurrence of an intracluster hydration reaction NO2+ + 2H(2)O --> HNO3 + H3O+. Vibrational excitation of the cluster led to loss of HNO3 as the major channel, as well as loss of H2O. New vibrational bands appeared and were assigned to solvated H3O+ and to HNO3 ligands. With the H3O+ replacing NO2+ as the charge center, the clusters rearranged to form H3O+(H2O)(2)(HNO3). Upon addition of a fifth water molecule, the hydronium ion was further stabilized by completion of the first hydration shell, leaving HNO3 in the second shell.