7H-Benzocyclohepten-7-one ( 1) reacts with oxalyl bromide to yield 7-bromobenzocycloheptenylium bromide ( 4a), a stable carbenium salt. In contrast, the reaction of oxalyl chloride or phosgene with 1 produces a covalent compound, 7,7-dichloro-7H-benzocycloheptent ( 3b), which ionizes in liquid sulfur dioxide to 7-chloro-benzocycloheptenylium chloride ( 4b). Nucleophiles are preferably added to C-5 ( C-9) of cation 4a; the resultant products are 7-bromo-5H-benzocycloheptenes ( 7b-h). The HMO model shows that the positive charge density in 4 is highest at C-5 ( C-9) thus favoring formation of addition products at this position under conditions of kinetic control. In those cases favorable to reversible addition, the competitive nucleophilic substitution at C-7 is the main reaction. The latter reaction occurs between 4a and ϱ-toluidine or N-methylaniline which produces 7-arylamino-benzocycloheptenylium bromides 13a,b. A further example is the hydrolysis of 4a and 3b to form 1.