Owing to the practical interest of the acid catalyzed isomerization reactions of hydrocarbons, the mechanism of the branching rearrangements of C4H9+ and C5H11+ carbenium ions has been studied theoretically using ab initio methods which include electron correlation and extended basis sets. It has been found that the protonated cyclopropane-type species does not appear as a common intermediate for these reactions, since it is a transition state and not a minimum on the potential energy surfaces studied. In the case of C4H9+ cation, the protonated methyl-cyclopropane ring is the transition state for the carbon scrambling reaction in the secondary n-butyl cation, while the isomerization of n-butyl cation into t-butyl cation occurs via a primary cation. The activation energies calculated assuming this mechanism are in very good agreement with those obtained experimentally. For the branching rearrangement of n-pentyl cation two reaction paths have been considered. In the first one the secondary n-pentyl cation is converted through the 1,2-dimethyl-cyclopropane ring into the secondary 3-methyl-2-butyl cation, which is converted into the I-pentyl cation by a 1,2-hydrogen shift. In the second one the secondary n-pentyl cation is directly converted into the t-pentyl cation through a primary monobranched cation. Comparison of the calculated activation energies for both mechanisms with the experimental value indicate that this reaction does not occur via the primary cation as was the case for n-butyl cation, but occurs via the protonated 1,2-dimethyl-cyclopropane ring.