Novel π-systems, three benzocyclohepta [a] azulenylium ions, 7a-c, are synthesized, and their stability and properties have been characterized in terms of the position of the benzo- annulation and compared with those of the parent cyclohepta [a] azulenylium ion 4. Benzocyclohepta [6, 7-a] azulenylium ion (7a)(p K R+= 7.3, E red=-0.567 V vs Ag/Ag+) and benzocyclohepta [6, 5-a] azulenylium ion (7b)(p K R+= 5.1, E red=-0.482 V vs Ag/Ag+), ...