Steady-state and time-resolved fluorescence techniques were employed to study the excited state proton transfer (ESPT) to water and D2O of a new photoacid, phenol benzoate cyanine picolinium salt (BCyP). We found that the ground-state pK(a) is about 6.5, whereas the excited-state pK(a)* is about -4.5. The ESPT rate constant, k(PT), to water is similar to 0.5 X 10(12) s(-1) (tau(PT) approximate to 2 ps) and in D2O the rate is 0.33 X 10(12) s(-1). We determined that the BCyP photoacid belongs to the third regime of photoacids, the solvent-controlled regime.