AbstractTo clarify the penultimate effect in the stereospecific polymerization by ionic catalysts, the relationship between the steric structure of the polymer and polymerization conditions was studied for α‐methylstyrene (αMS), methyl vinyl ether (MVE), and methyl methacrylate (MMA) from our result and literature results. Two possible probabilities of an isotactic addition considering the penultimate effect, Pii and Psi, are compared. We define the α value as Psi = αPii. The greater the deviation of the α‐value is from unity, the greater the penultimate effect. It is shown that the α value in a given monomer is constant for a wide range of tacticity in the various conditions. In αMS, the α value is unity and there is no penultimate effect. Otherwise, in the MVE‐BF3.‐O(C2H5)2 system, α is 0.7 and the penultimate effect is present. When MMA is polymerized by alkali alkyl, the α value is changed by the kind of alkali metal, that is, α values in toluene solution are 0.45, 0.65, and 0.75 for Li, Na, and K, respectively.