Values of k(o) = 8.0 x 10(-3) s(-1) and k(H) = 2.5 x 10(-2) M-1 s(-1), respectively, were determined for the spontaneous and the acid-catalyzed cleavage of 4-methoxybenzyl fluoride (1-F) to form the 4-methoxybenzyl carbocation (1(+)). Values of k(F) = 1.8 x 10(7) M-1 s(-1) and k(HF) = 7.2 x 10(4) M-1 s(-1) were determined for addition of F- and HF to 1(+) for reaction in the microscopic reverse direction. Evidence is presented that the reversible addition of HF to 1(+) to give 1-F + H+ proceeds by a concerted reaction mechanism. The relatively small 250-fold difference between the reactivities of fluoride ion and neutral HF toward 1(+) is attributed to the tendency of the strong aqueous solvation of F- to decrease its nucleophilic reactivity and to the advantage for the concerted compared with the usual stepwise pathway for addition of HF. There is no significant stabilization of the transition state for cleavage of 1-F from general acid catalysis by 0.80 M cyanoacetate buffer at pH 1.7. The estimated 3 kcal/mol larger Marcus intrinsic barrier for heterolytic cleavage of 1-F than for cleavage of 1-Cl is attributed to a lag in the development at the transition state of the ca. 30 kcal/mol greater stabilizing solvation of the product ion F- compared with Cl-. The decrease in the electronegativity of X along the series X = F, OH, Cl is accompanied by a ca. 10(10)-fold increase in the carbon basicity compared with the proton basicity of X-.