Kinetics of excited-state proton transfer from 6-hydroxynaphthalene-1-sulfonic acid to water in n-propanol/water mixtures has been studied at room temperature. The fluorescence decay is single exponential in n-propanol and becomes bi-exponential in n-propanol/water mixtures. The proton transfer occurs in the binary mixtures above the critical water concentration which is 4 M. A water dimer has been found to be an effective proton acceptor. The presence of reverse reaction indicates the need of the stable solvation to excited conjugate base aftermath of the proton transfer. (C) 2011 Elsevier B.V. All rights reserved.