Rate constants for proton transfer to the C 4H 8 isomers, cis- and trans-2-butene, 1-butene, and methylcyclopropane from various proton donors (such as CH 3CNH +, CH 3CHOH +, AsH 4 +, H 3S + and H 3O +) are reported, and the structures of the product C 4H 9 + ions have been determined. It is seen that when proton transfer to form the sec-butyl ion is endothermic for a particular reactant pair, proton transfer to form the tert-butyl ion may be observed but only if the [M · i-C 4H 9 +] complex (corresponding to the transition state for the rearrangement of the C 4 moiety from an unbranched to a branched structure in the non-covalently bonded complex) is energetically accessible to the separated reactants. When proton transfer to form the sec-butyl ion is exothermic, it will be the exclusive proton transfer channel if the transition state for the isomerization process is energetically unaccessible. In some mixtures, on the other hand, both sec-butyl and tert-butyl ions are formed as products. The probability of rearrangement in the complex is greater, the greater the dipole moment of the M species, since a larger dipole moment is associated with a deeper well depth for the ion/molecule complex, and hence, a lower energy for the “transition state complex”. When proton transfer to form an unrearranged sec-butyl product ion is highly exothermic ( > 10 kcal mol −1), that channel will predominate over the rearrangement channel, even if the transition state for the isomerization is energetically favorable. Similar models are shown to explain results from the literature on structure retention or ring opening in protonated cyclopropane ions. Results on the protonation of 2-pentene indicate that rearrangement of sec-C 5H 11 + to tert-C 5H 11 + occurs in the ion/molecule complex when proton transfer to give a separated sec-C 5H 11 + product is endothermic.