Alkyl cations of the form. CH3C(+)HR have been investigated using high-level quantum chemical methods to study the influence of alpha-substituents R (R = H, CH3, CH=CH2, C-CH, F, and Cl) on cation geometries and relative energies with respect to the neutral precursors CH3CH2R. The results of density-functional B3LYP computations with a variety of basis sets were compared with MP2, MP4, QCISD(T), and CBS-Q model chemistry results and benchmarked against experimental data. The results show that geometrical features, are already accurately described using B3LYP/6-311G(d,p) or MP2/6-311G(d,p). For a systematic study of the energetics of ct-substitution on alkyl cations, B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) computations form a useful compromise between accuracy (average deviation within 1 kcal/mol of the experimental error) and computational efficiency. The electronic structures of these species and their precursors CH3CH2R were studied using both natural bond orbital (NBO) and Atoms-in-molecules (ATM) analyses. These analyses clearly show that the electron-donating power of alpha-substituents at (partially) positively charged carbon atoms does not correlate well with the thermochemical stabilization of cations provided by such substituents.