Dihalocarbenes add regioselectively to aryl-substituted benzocyclopropenes to produce dihalobenzocyclobutenes. The regioselectivity of addition is not due to steric effects but depends on the electronicdonor or acceptor ability of the substituent. B3LYP/6-31G* calculations show preferential :CCl2 additionto substituted benzocyclopropene through electrophilic attack on the benzocyclopropene π-system (Ea =1.1−2.4 kcal/mol) rather than C−C σ-bond insertion into the cyclopropenyl moiety (Ea = 5−24 kcal/mol). π-Addition proceeds regioselectively through a single transition state to xylylene intermediates ordirectly to benzocyclobutene products.