The gas-phase acidity of quadricyclane, C7H8, has been determined using a flowing afterglow-selected ion flow tube at room temperature. Measurements of forward and reverse rate constants for the proton transfer reactions B- + C7H8; reversible arrow C7H7- + BH where BH = NH3, CH3NH2, and C6H6 give Delta(acid)G(300)(C7H8) = 394.7 +/- 0.8 kcal/mol and Delta(acid)H(300)(C7H8) = 403.0 +/- 1.1 kcal/mol. Combining this value with the electron affinity, 0.868 +/- 0.006 eV, of the quadricyclyl radical, gives 109.4 +/- 1.3 kcal/mol for the C-H bond dissociation energy of quadricyclane at Cl. Reactions of quadricyclanide ion with various reagents have been studied, and branching ratios of product channels have been determined. These results, along with ab initio calculations and a companion photoelectron spectroscopy study, indicate that there are two isomers of quadricyclanide ion which have similar energies.