Owing to the practical interest of the acid-catalyzed isomerization of n-butane and l-butene, the mechanism of the isomerization and scrambling reactions of the n-butyl cation has been studied theoretically using ab initio methods which include electron correlation and extended basis sets. It has been found that the protonated cyclopropane ring does not appear as a common intermediate for carbon scrambling and branching isomerization of the n-butyl cation, since it is a transition state and not a minimum on the potential energy surface. The transition states for both reactions have been determined and the activation energies calculated. These values are in very good agreement with those obtained experimentally.