The reactions of the water solvated ammonia radical cation [NH 3 ⅐ϩ , H 2 O] with a variety of aldehydes and ketones were investigated. The reactions observed differ from those of low energy aldehydes and ketones radical cations, although electron transfer from the keto compound to ionized ammonia is thermodynamically allowed within the terbody complexes initially formed. The main process yields an ammonia solvated enol with loss of water and an alkene. This process corresponds formally to a McLafferty fragmentation within a complex. With aldehydes, another reaction can take place, namely the transfer of the hydrogen from the CHO group to ammonia, leading to the proton bound dimer of ammonia and water, and to the NH 4 ϩ cation. Comparison between the available experimental results leads to the conclusion that the McLafferty fragmentation occurs within the terbody complex initially formed, with no prior ligand exchange, the water molecule acting as a spectator partner. (J Am Soc Mass Spectrom 2004, 15, 966 -971) T he study of the unimolecular and bimolecular reactivity of solvated radical cations requires access to such kind of species. Of course, by direct reactions between an ion and a solvent molecule in the cell of an ion cyclotron resonance mass spectrometer (FT-ICR), an encounter complex is formed in which reactions can take place. However, the solvated radical cations formed in this way possess a high energetic content (at least equal to the solvation energy) and are too short lived to be isolated and studied. Furthermore, since these species are hot ions, their fragmentations correspond to the unimolecular reactions which are entropically favored. Therefore, the reactions observed are not necessarily the same as those of long lived solvated radical cations, as demonstrated in the case of the CH 2 CHOH ⅐ϩ /CH 3 CHO system [1], where the acetaldehyde solvated enol ion is produced by reaction of the enol ion with paraldehyde (eq 1). In this case, the hot intermediate reacts by H ϩ transfer, whereas low energy collision induced dissociation of the solvated complex leads to H ⅐ abstraction [1]. CH 2 CHOH ⅐ϩ ϩ (CH 3 CHO) 3 3 [CH 2 CHOH ⅐ϩ , CH 3 CHO] ϩ (CH 3 CHO) 2 (1)