Reaction of CO 2 with o-, m-, and p-benzyne radical anions in the gas phase produces o-, m-, and p-dehydrobenzoate radical anions, respectively. The (oxygen) gas-phase basicities of these ions, which are equivalent to the gas-phase acidities of the corresponding dehydrobenzoic acid radicals, ΔG a c i d (o-, m-, or p-C 6 H 4 CO 2 -H), have been determined with a flowing afterglow-triple quadrupole apparatus by means of the kinetic method. The measured values are (in kcal mol - 1 ): ΔG a c i d (o-C 6 H 4 CO 2 H) = 330.4 ± 0.4, ΔG a c i d (m-C 6 H 4 CO 2 H) = 330.2 ± 0.4, and ΔG a c i d (p-C 6 H 4 CO 2 H) = 331.6 ± 0.4 kcal mol - 1 . All three radicals are more acidic than benzoic acid (ΔG a c i d = 333.1 ± 2.0 kcal mol - 1 ). The measured gas-phase acidities for the meta and para isomers suggest values for the resonance-effect substituent constant, σ R , and the field/inductive effect substituent constant, σ F , for a phenyl radical site of -0.47 and 0.57, respectively. This classifies a phenyl radical site as a strong inductive withdrawing, and strong resonance donating substituent. Density functional calculations of the gas-phase acidities of dehydrobenzoic acids are in good agreement with the experimental results. The increased acidities of the dehydrobenzoic acids are shown to arise from a balance between the electron withdrawing effect of the electronegative radical site, and a compensating polarization of the π system which mimics the effect of a resonance donor group located at the radical carbon.