The kinetic acidities of members of a series of halomethyltrimethylammonium salts (Me3N⁺CH2X, BF4^â) have been measured under conditions of ^âOD/D2O at 50â °C, with the rate constants being found to increase Xâ =â Clâ <â Brâ <â I. The observed trend is rationalized on the basis of the polarizabilities and electron repulsion factors of the respective halogen substituents. The observed rate constants indicate a lowering of the Î Gâ ^{â ¡} for deuterium/hydrogen exchange of 12â 13 kcal mol^{â 1} due to the presence of the α-halo substituents. The kinetic acidity of chloromethyldimethylsulfonium BF4^â was â 10⁶ that of the analogous chloromethylammonium salt.