Steady-state and time-resolved fluorescence techniques were employed to study a superphotoacid with a pK(a)* of similar to-7, the chlorobenzoate phenol cyanine picolinium salt (CBCyP) in acetonitrile-water mixtures. We found that the time-resolved fluorescence is bimodal. The amplitude of the short-time component depends on chi(water); the larger chi(water), the greater the amplitude. We found that the excited-state proton transfer (ESPT) rate constant, k(PT), is >= 5 X 10(12) s(-1) in mixtures of chi(water) >= 0.08, whereas in neat water, k(PT), = 6 X 10(12) s(-1). The long-time component has a lifetime of 50 ps at chi(water) = 0.75. We attribute this time component to the CBCyP molecules that are not hydrogen-bonded to H2O clusters. The results suggest that the ESPT rate constant to water in acetonitrile-water mixtures depends only slightly on the water cluster size and structure surrounding the CBCyP molecule. We attribute the independence of the ESPT rate on the average water-cluster size to the large photoacidity of CBCyP. QM TD-DFT calculations found that in the excited-state the RO-(S-1) species that is formed by the ESPT process is more stable than the ROH(S-1) species by -5 kcal/mol when four water molecules accept the proton, and when six water molecules accept the proton, the RO-(S-1) drops to -10 kcal/mol. The calculations show that energy stabilities are kept constant in implicit CH3CN-H2O solvent mixtures of dielectric constant of epsilon >= 45.