The simple cleavage of the CC bond in tetramethyl ethylenediamine molecular ions yields two fragments that are identical but for the positions of the charge and radical; nonetheless, the reactions of deuterium labeled analogs are accompanied by substantial secondary kinetic isotope effects. The underlying transition state zero-point vibrational energy differences depend particularly on the properties of the incipient radicals rather than on those of the charge-retaining products. The α-secondary effects arise primarily from vibrations related to deformation of the product-CH 2 N-and-CD 2 N-groups when the reactant is labeled at the central CC bond, whereas CH/CD stretching vibrations are the origin of the γ-secondary effects observed for reactants with labeled methyl groups. These zero-point energy differences are mainly due to hyperconjugative interactions (Bohlmann shifts); the secondary isotope effects on the dissociation of protonated amine dimers have a similar origin. # Dedicated to the memory of Nico M. M. Nibbering, for many years an important figure in European mass spectrometry research.