The rate of reaction of several p‐substituted benzyl bromides with lithium thiophenoxide (kphs) and lithium methoxide (KMeO) in methanol have been measured polarographically. In both series of reactions, the reactivity increases on introduction of electron releasing and electron attracting substituents. This is explained by a change in the transition state structure from an essentially ionic form, to a more covalent form stabilised by conjugation between the benzene ring and the pseudo‐π orbitals forming the reaction coordinates. The ratio kPhs/kMeoincreases with the HAMMETTσ value for the substituent in the benzyl bromide, showing that the selectivity of the nucleophile increases with the extent of covalent bond formation in the transition