Fluorescence kinetic constants are measured as functions of wavelength, pH, hydrogen isotope, and temperature to understand proton transfers involved in the excited-state tautomerization of aqueous 6-hydroxyquinoline, Not only the detailed mechanism but also the individual catalytic roles of H2O, H3O+, and OH-in the tautomerization reaction have been determined. The variation of the relative contribution with pH is determined for each reaction pathway as well. Solvent structural dynamics is found to be important in the deprotonation of the enol group and in the migration of a hydrated proton cluster.