A flowing afterglow-tandem mass spectrometer has been used to measure the 298 K gas-phase basicity (GB) of sulfuric acid. The kinetic method gives GB(H2SO4) = 683 +/- 5 kJ/mol, which corresponds to a proton affinity of 714 kJ/mol. Reaction rate measurements give GB(H2SO4) greater than or equal to 680 +/- 3 kJ/mol, while high-level ab initio calculations give a proton affinity of 720 kJ/mol, in good agreement with the kinetic method results. These GB values are higher than the previously bracketed limits. The GB is higher than that for water and lower than that for nitric acid, the other two important components of polar stratospheric clouds.