The potential energy surface of protonated sulfuric acid has been studied using a flowing afterglow tandem mass spectrometer and ab initio calculations, Energy-resolved collision-induced dissociation of H3SO4+ and H3O+.SO3 indicates that decomposition of H3SO4+ has a substantial barrier of 224 +/- 13 kJ/lmol, while H3O+. SO3 is bound by 74 +/- 15 kJ/mol. Collision-induced dissociation of H3SO4+.H2O and the reaction of H3SO4+ with H2O show that water can catalyze the dissociation of H3SO4+ The overall potential energy surface is remarkably similar to that for the addition of H2O to SO3, indicating that the additional proton does not have a major effect on the reaction. The experimental and computational results are in excellent agreement fur all species where sulfur is tricoordinate. When the sulfur is tetracoordinate, very large basis sets appear to be necessary for accurate energetics.