The unimolecular dissociation of protonated ethylamine, [CH3CH2NH3](+), 1, has been examined by both mass spectrometric techniques and molecular orbital calculations. Metastable dissociations of various deuterated species demonstrate that the loss of ethene from 1 involves specifically a transfer of a hydrogen atom from the methyl to the NH2 group. A very limited scrambling (4%) of the hydrogen atoms of the ethyl moiety is observed. Ab initio molecular orbital calculations have been performed at the MP4SDTQ/6-311G**//MP2/6-31G*+ZPE level of theory to examine the mechanism of the unimolecular dissociation of ions 1. It appears that 1 loses a molecule of ethene after a rate-determining isomerization into a proton-bound complex : [C2H4 ...-NH4](+), 2. The marginal hydrogen scrambling is accounted for by 1,2-H shifts on the ethyl group inside a second, loosely bonded, complex involving an ethyl cation and a molecule of ammonia : [C2H5 ... NH3](+), 3.