Quantum-chemical calculations were carried out on the mechanism of the zeolite-catalyzed hydrocarbon skeletal isomerization via the cyclopropane ring intermediates. According to the B3LYP/6-31G* calculations, formation of cyclopropane from surface alkoxy species in zeolites occurs via a transition state whose hydrocarbon part resembles a corner-protonated cyclopropane (corner-PCP) ring. Two conformations of the transition state found differ in the orientation of the PCP portion with respect to the acid site. The activation energy for the cyclopropane ring closure reaction is found to be rather sensitive to the use of planar symmetry constraints and to the level of calculations and less sensitive to the level of the geometry optimization. Calculations on the mechanism of the carbon isotope scrambling in the free 2-propyl cation were also performed, at several theory levels up to the Gaussian-2 model. The relatively stable intermediates of this superacid-catalyzed reaction are carbocations, in contrast to the zeolite-catalyzed isotope scrambling where the relatively stable intermediates are surface alkoxy species with the corner-protonated cyclopropane as a high-energy transition state.