An unusually large tunneling effect on the relaxation rate of photon-perturbed ground-state protropic equilibrium is observed in the near-room-temperature range by measuring transient absorption kinetic profiles of aqueous 7-hydroxyquinoline solutions. The observed proton-transfer rate is analyzed into H3O+- and OH--catalyzed components that are diffusion-controlled and into a water-catalyzed component. The difference of activation energies, (E(a),2(H2O) - E(a),1(H2O)) and the ratio of Arrhenius preexponential factors, A1(H2O)/A2(H2O) are 15 kJ mol(-1) and 0.008, respectively for the water-catalyzed rate constant, indicating a large tunneling effect, The observation of a small kinetic isotope effect (similar to 3.0 at room temperature) despite the large tunneling contribution suggests that the variational effect and early transition state are also important.