The structures and energies of the carbocations C4H7+ and C5H9+ were calculated using the ab initio method. The C-13 NMR chemical shifts of the carbocations were calculated using the GIAO-CCSD(T) method. The pi sigma-delocalized bisected cyclopropylcarbinyl cation, 1 and nonclassical bicyclobutonium ion, 2 were found to be the minima for C4H7+ at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level the structure 2 is 0.4 kcal/mol more stable than the structure 1. The C-13 NMR chemical shifts of 1 and 2 were calculated by the GIAO-CCSD(T) method. Based on relative energies and 13 C NMR chemical shift calculations, an equilibrium involving the 1 and 2 in superacid solutions is most likely responsible for the experimentally observed 13 C NMR chemical shifts, with the latter as the predominant equilibrating species. The alpha-methylcyclopropylcarbinyl cation, 4, and nonclassical bicyclobutonium ion, 5, were found to be the minima for C5H9+ at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZJ/MP2/cc-pVTZ + ZPE level ion 5 is 5.9 kcal/mol more stable than the structure 4. The calculated C-13 NMR chemical shifts of 5 agree rather well with the experimental values of C5H9+.