Unimolecular deamination of protonated alpha,omega -alkanediamines (1,2-ethanediamine (1), 1,3-propanediamine (2), 1,4-butanediamine (3), and 1,5-pentanediamine (4)) in the gas phase has been examined by tandem mass spectrometry experiments including metastable ions decompositions and collisional activation techniques and molecular orbital calculations up to the G2(MP2,SVP) level. For all the protonated molecules, only one unimolecular dissociation channel, leading to the formation of a protonated cyclic amine via an internal nucleophilic substitution, was detected. The hydride ion transfer from the alpha carbon to the omega position is not competitive with the internal nucleophilic substitution. This has been found to be the result of a large critical energy for this latter reaction.