Steady-state and time-resolved fluorescence techniques were used to study excited-state proton transfer (ESPT) to water of the reversible photoacid 2-naphthol-8-sulfonate (2N8S) in acetonitrile/water mixtures. In acetonitrile-rich mixtures, up to chi(water) <= 0.12, we found a slow ESPT process on the order of nanoseconds. At Xwater chi(water) approximate to 0.15, the RO- fluorescence band intensity is at the minimum, whereas at chi(water) approximate to 0.030, it is at the maximum. The steady-state fluorescence spectra of these mixtures show that the intensity of the RO- fluorescence band at chi(water) approximate to 0.030 is about 0.24 of that of the ROH band. We explain this unusual phenomenon by the presence of water clusters that exist in the acetonitrile-rich CH3CN/H2O mixtures. We propose that a water bridge forms between the 2-OH and 8-sulfonate by preferential solvation of 2N8S, and this enables the ESPT process between the two sites of the molecular structure of 2N8S. In mixtures of chi(water) >= 0.25, the ESPT process takes place to water clusters in the bulk mixture. The higher the chi(water) in the mixture, the greater the ESPT rate constant. In neat water, the rate constant is rather small, 4.5 X 10(9) s(-1). TD-DFT calculations show that a single water molecule can bridge between 2-OH and 8-sulfonate in the excited state. The activation energy for the ESPT reaction is about 9 kcal/mol, and the RO-(S-1) species is energetically above the ROH(S-1) species by about 1.6 kcal/mol.