Structures of the tert-pentyl cation (C₅H₁₁⁺) and its protonated dication (C₅H₁₂²⁺, isopentane dication) were studied using ab initio methods at the MP2/cc-pVTZ level. Both C–C and C–H hyperconjugatively stabilized structures 1 and 2, respectively, were found to be minima on the potential energy surface (PES) of the tert-pentyl cation. Structure 1 was computed to be about as stable as structure 2 (slightly more stable by 0.5 kcal mol^–1). Inter-conversion between 1 and 2 through transition state 3 has a kinetic barrier of only 1.5 kcal mol^–1. The C–H protonated form (H₃C)₂C⁺CH₂CH₄⁺4 was found to be the global minimum for the protonated tert-pentyl dication. Charges and ¹³C NMR chemical shifts of the dication 4 were calculated and compared to those of monocation 1 to study the effect of the additional charge in the dication.