To explore how a structured excess charge distribution affects the hydration of an anion, we report mid-IR, argon predissociation spectra for the hydrated superoxide cluster anions, O-2(-). (H2O)(n), 1 less than or equal ton less than or equal to5. This size range was chosen to establish the evolution of the structures through the putative shell closing [Weber , Science 287, 2461 (2000)] for superoxide hydration at the tetrahydrate. Whereas the observed bonding motifs for n less than or equal to4 are those of single water molecules and dimeric subclusters bound to the ion, the pentahydrate spectrum displays strong bands in the region typically associated with ring modes of the water trimer. The present results reinforce the conclusion that the tetrahydrate adopts an especially robust structure in which each water molecule forms a single ionic H bond to one of the lobes of the pi* highest occupied molecular orbital in superoxide. (C) 2001 American Institute of Physics.