The loss of HBr from 2-bromobutane radical cation, 1, was investigated by means of mass spectrometry techniques. In the gas phase, the spontaneous unimolecular reaction of metastable ion 1 yields only a C4H8.+ (m/z 56) fragment and neutral HBr. In all the deuterated ions the loss of HBr is always strongly dominant, the loss of DBr remaining under 1%. Ion-molecule reactions indicate that the C4H8.+ ions correspond to a mixture in which ionized 2-butene and methylcyclopropane were detected. These results are discussed in the light of calculations carried out by using high-level density functional theory techniques. For this purpose we have located the relevant stationary points of the [H-9, C-4, Br](.+) potential energy surface (PES). The geometries and the vibrational frequencies were obtained at the B3LYP/6-31G* level, and for the final energies a B3LYP/6-311+G(3df,2p) approach was used. From these data, it is proposed (i) that the loss of HBr involves 1,3-and 1,4-elimination reactions and several elimination pathways involving tunneling mechanisms and (ii) that the dissociation is preceded by an extensive H-exchange whose mechanism involves the intermediacy of ionized methylcyclopropane weakly bonded to a HBr molecule.