The new, bright red compounds [DBS][B(C₆F₅)₄], [DBS][BCl(C₆F₅)₃], [DBS][B(C₆F₅)₃–OH⋯H₂O–B(C₆F₅)₃] and [DBS][B(CH₂CMe₃)(C₆F₅)₃] (DBS = C₁₅H₁₁, dibenzosuberenyl, dibenzotropylium, dibenzo[a,e]tropylium or dibenzo[a,d]cycloheptenocarbonium) have been synthesized and characterized spectroscopically and, except for [DBS][B(CH₂CMe₃)(C₆F₅)₃], crystallographically. The crystal structures show that the DBS⁺ carbocation is planar and suggest pronounced delocalization of the positive charge over part of the seven-membered ring, in particular the CH carbon and the carbon atoms α and β to it. The electrophilic DBS⁺ carbocation activates alkene polymerization catalyst precursors of the type Cp′₂ZrMe₂, its efficacy being comparable to that of the well known but sterically more hindered trityl carbocation, Ph₃C⁺. As with the Ph₃C⁺ ion, DBS⁺ does not initiate carbocationic polymerization of isobutene in dichloromethane directly, but rather by reacting with trace amounts of water to generate protons.