The nature of the process 'hydrogen atom' or 'proton' transfer reaction in neutral and single positive ions of salicylic acid, o-hydroxybenzaldehyde, and salicylaldimine has been studied using the B3LYP density functional method. Single-point calculations at the CCSD(T) level have also been performed. For neutral systems, the reaction corresponds to a proton transfer process coupled with an important electronic transfer, which leads to a compound with nonzwitterionic character. A similar behavior is observed for ionized π radical cations. In ionized σ states, however, the process is that of a hydrogen transfer. In all cases, π ionization strengthens the initial H-bond and favors the reaction. For some of the systems the proton transfer occurs spontaneously.