The isomer distributions and the rates relative to those of the parent heterocyclic compounds for several electrophilic substitutions of benzo[b]furan and benzo[b]thiophen have been determined. Surprisingly, although the orientation of the substitution in the two bicyclic systems is different, the effect caused by ‘annelation’ on the reactivity of the α- and β-positions is substantially similar in the two rings: the reactivity of the α-position is always decreased by a similar factor and the reactivity of the β-position is (with two exceptions) increased, in both systems.