A threshold collision-induced dissociation (TCID) study was performed on the n-butylbenzene cation, investigating the competitive dissociation leading to propyl and propene elimination. Ab initio calculations at the B3LYP/6-311++G(2d,2p)//B3LYP/6-31G* level were performed on the system. Reaction path calculations were performed for both dissociation channels to provide details of the dissociation mechanisms and to identify rate-determining transition states. Unimolecular dissociation rates as a function of energy from literature PEPICO and PD-MIKES experiments are modeled using RRKM statistical unimolecular decay theory to select the best calculated transition states for the two dissociation channels. The effects of kinetic and competitive shifts on the CID threshold determinations are investigated and explained using a model that incorporates RRKM theory. Final analysis of the TCID data yields 0 K dissociation energies of 1.70 +/- 0.09 eV for the propyl elimination channel and 1.28 +/- 0.06 eV for the propene elimination channel. On the basis of the current and previous studies, we identify procedures for selecting appropriate transition states when the feedback obtained from experimental dissociation rates is not available.