Various deuteriated isotopologs of ethylisopropylammonium ion have been subjected to infrared laser multiple-photon decomposition in an ion cyclotron resonance (ICR) FT-MS spectrometer. The intermediate product of the decomposition is a complex of 2-propyl cation and ethylamine which undergoes an internal proton transfer to give propylene and ethylammonium ion. Although this reaction takes places place very close to threshold, the primary kinetic isotope effect is very small (less than or equal to 1.15), consistent with the proposal that the choice of reaction channel takes place at large separation with essentially no bond breakage or formation.