The unimolecular dissociation of n-propanol molecular cation (1) has been investigated theoretically. Density functional theory (DFT) molecular orbital calculations have been performed at the UB3LYP/aug-cc-pVQZ//UB3LYP/6-31+G(d) level to obtain pathways for water and hydrogen atom loss from 1. On the basis of the DFT results, the rate-energy dependences have been calculated for water and hydrogen atom loss from various deuterated analogues of 1 by Rice–Ramsperger–Kassel–Marcus modeling. The large kinetic deuterium isotope effects on the metastable water and hydrogen atom loss reported previously are rationalized from the obtained rate-energy dependences.