The dissociation constants of 29 α-substituted 4-methylpyridinium ions in water at 25 °C have been measured by potentiometric or spectrophotometric methods. Correlation of the logKvalues with σIvalues gives, forn = 13, slope ρ = 2.922, intercept logK0 = 5.93, correlation coefficientr = 0.990. Effective σIvalues evaluated from the dissociation of substituted 4-picolinium ions are in good agreement with those calculated from dissociation of substituted 2- and 3-picolinium ions and with literature values. pKvalues of the 4-pyridinylmethylammonium ion and itsN-methyl andN,N-dimethyl derivatives are 8.22, 8.48, and 7.63, respectively.