The sputtering of condensed-phase ammonium nitrate yields an extensive distribution of cluster ions with the general formula [NH4NO3)(n)NO3]-, where n 3 to >40. Only a very low relative abundance of the n land the n = 2 species are detected by mass spectrometry. Collision-induced dissociation experiments suggest that [NH4NO3)NO3]- rearranges to lose NH3 and form [H(NO3)(2)](-), which is detected at high relative abundance. Gradient-corrected density functional calculations show that there is strong hydrogen bonding in NH4NO3 and essentially no hydrogen bonding in [NH4NO3)NO3](-). Thus, with a relatively small amount of internal energy the latter can rearrange to lose a neutral ammonia molecule.