The C4H9+ potential energy surface (PES) is investigated using high level ab initio molecular orbital theory. All structures were fully optimized at Hartree-Fock (HF) and correlated levels (HF/6-31G*, MP2(full)/6-31G*, and MP2-(full)/6-31G**) followed by single point energy calculations at MP4sdtq/6-31G**//MP2(full)/6-31G**. The C(s) 1, C3h 2, and C3v 3 forms of the tert-butyl cation were investigated. At our final level, MP4sdtq/6-31G**//MP2/6-31G** + ZPE, 2 is more stable than 1 by only 0.1 kcal/mol. Nevertheless, better agreement between C-13 chemical shifts computed using the IGLO (individual gauge for localized molecular orbitals) method, and experimental data were found with 1. The stability and structure of tert-butyl cation is interpreted in terms of the bending of CH bonds due to their hyperconjugative interaction with the formally ''vacant'' p-orbital on the carbenium center. At our final level, 3 is 1.1 kcal/mol higher in energy than 2. Of the rive 2-butyl cation structures investigated, the symmetrically hydrogen-bridged minimum 5 (C2 Point group) is the more stable. However the C1 methyl-bridged alternative 4 lies only 0.4 kcal/mol higher in energy. The IR spectrum of the H-bridged form 5 computed at MP2/6-31G* shows a 2100 cm-1 frequency which is characteristic in the otherwise sparcely populated 2000-cm-1 region. This is in good agreement with a peak at 2175 cm-1 in the IR spectrum of the 2-butyl cation measured at -125-degrees-C in a SbF5 matrix. The computed spectrum of the alternative methyl bridged 2-butyl cation 4 has no frequency in the 1600-2800 cm-1 region. The classical open chain isomer 7 is 2.3 kcal/mol less stable than 5. H- and C-scrambling in the 2-butyl cation occurs via the edge protonated methylcyclopropane transition structure 9, calculated to be 8.5 kcal/mol less stable than the minimum 5. The activation barrier for the 2-butyl cation rearrangement into the tertiary isomer was calculated to be 19.6 kcal/mol in acceptable agreement with the 18 kcal/mol value derived from DNMR measurements. Besides these minima, two additional minima were found, i.e., edge protonated cyclobutane adopting a twisted (point group C2) geometry 14 and a protonated methylcyclopropane 10. However, protonated cyclobutane (14) is unfavorable energetically (35.9 kcal/mol less stable than the tert-butyl cation), while protonated methylcyclopropane (10) is only 8.6 kcal/mol higher in energy than the 2-butyl cation, The final relative stability ordering of butyl cations is predicted theoretically to be 0, 13.4, 22.0, 33.4, 35.7, and 35.9 kcal/mol for the tert-butyl (1), 2-butyl (5), protonated methylcyclopropane (10), isobutyl (12), 1-butyl (13), and protonated cyclobutane (14) ions, respectively. However, the isobutyl (12) and 1-butyl (13) cations are transition structures (rather than minima). The infrared spectra theoretically predicted at the MP2/6-31G* level also are reported.