Benzocyclobutadiene radical anion (5) was synthesized in the gas phase by three independent approaches: collision-induced dissociation (CID) of 1,2-benzocyclobutenedicarboxylate, reaction of 2-trimethylsilyl-1 -benzocyclobutenyl anion with neopentyl nitrite followed by CID of the resulting beta -nitroso carbanion intermediate, and reaction of the same beta -silyl carbanion with molecular fluorine. The proton affinity and electron binding energy of 5 were measured (0.32 +/- 0.05 eV and 368 +/- 2 kcal mol(-1), respectively) and combined in a thermodynamic cycle to obtain the heat of hydrogenation (49 +/-4 kcal mol(-1)) and the heat of formation (97 +/- 4 kcal mol(-1)) of benzocyclobutadiene (1), These results were compared to model compounds as well as MP2 and B3LYP calculations in order to assess the antiaromatic destabilization energy of 1. Based upon our data, a predicted heat of formation for dyclobutadiene (102 kcal mol(-1)) was obtained. This work demonstrates the utility of dicarboxylates as radical anion precursors and the electron as a protecting group.