We used steady-state and time-resolved fluorescence techniques to study the excited-state proton transfer (ESPT) and the nonradiative properties of two irreversible photoacids, 1-naphthol-4-sulfonate (1N4S) and l-naphthol-5sulfonate (1N5S). We found that the ESPT rate constant of 1N4S in water is 2.2 x 10(10) s(-1), whereas in methanol, it is smaller by about 3 orders of magnitude and is not observed. The ESPT process of 1N5S competes with a major nonradiative process of equal rate and k(pt)of 2.2 x 10(10) s(-1). in methanol-water mixtures of X-H2o = 0.2, the fluorescence lifetime of the ROH form of 1N5S is lower by a factor of 10 than that in pure methanol. In the steady-state fluorescence spectra of 1N5S in methanol-water mixtures, there are two iso-emissive points, one for X-H2o < 0.2 and one for X-H2o > 0.3. This large reduction in fluorescence intensity and the two iso-emissive points are explained by the existence of a mixed water-methanol bridge of about three molecules that connects the proton donor 1-OH with the 5-sulfonate in mixtures of X-H20 < 0.2. The bridge enhances both the ESPT and the nonradiative processes. For 1N4S in methanol-water mixtures at X-H2o approximate to 0.2, the reduction in the fluorescence lifetime is only by similar to 30%, and only one iso-emissive point exists in the steady-state fluorescence spectra for 0