C-13 NMR spectroscopy of the 2-methyl-2-butyl-1-C-13 cation (C-13-labeled tert-amyl cation) indicates that interchange of the inside and outside carbons occurs via a barrier of 19.5 +/- 2.0 kcal/mol. A plausible mechanism involves hydride migration in the proposed 2-pentyl cation 4 to form 3-pentyl cation 5. Via the protonated cyclopropane intermediate 6, which undergoes degenerate corner-to-corner hydride shift, the secondary 3-pentyl cation 5' with the label shifted to the central carbon atom is formed. The tert-amyl cation obtained from 5' in the reverse process has the C-13 label on an inside carbon atom. All intermediates and transition structures were located on the PES theoretically at the MP2/6-31 G(d,p) level of theory. The rearrangement rate of the doubly labeled tert-amyl cation (methyl-C-13-butyl-1-C-13 cation), followed by means of C-13 NMR, revealed that the process that interchanges inside and outside carbons has the highest barrier. Comparison of the initial rates revealed that isotopomer le arises considerably more slowly than other isotopomers, indicating that in the overall rearrangement process transition structure 5-TS has the highest energy.