The effects of α-substituents on alkyl and vinyl cations are studied using high-level ab initio calculations. The geometries, stabilities, and electronic properties of 27 alkyl cations and 27 vinyl cations with α-substituents are computed at the B3LYP/6-311+G(d,p), MP2/6-311+G(d,p), and CBS-Q levels. The substituents studied vary from strongly destabilizing (e.g., α-CN and α-CF3) to strongly stabilizing (e.g., α-OSi(CH3)3 and α-NH2). The calculations show that in the case of vinyl cations the stabilization provided by the α-substituents is larger by an average value of 4 kcal/mol than for the alkyl ones. This is the result of the intrinsically lower stability of vinyl cations (on average 17 kcal/mol). However, strong inductively donating or withdrawing α-substituents show different behavior. Because of the high amount of s character in the carbon−α-substituent bond in the vinyl cations (sp hybridized), more pronounced σ effects are found than in the corresponding alkyl cations, leading to lower stabilizat...