Steady-state and ultrafast time-resolved techniques were used to study a newly synthesized photoacid, phenol-carboxyether dipicolinium cyanine dye, QCy9. We found that the excited-state proton transfer (ESPT) to water occurs at the remarkably short time of about 100 fs, k(PT) approximate to 1 x 10(13) s(-1), the fastest rate reported up to now On the basis of the Forster-cycle, the pK(a)* value is estimated to be -8.5 +/- 0.4. In previous studies, we reported the photoacidity of another superphotoacid, the QCy7 for which we found an ESPT rate constant of similar to 1.25 x 10(12) s(-1), one eighth that of the QCy9 compound. We found a kinetic isotope effect of the ESPT of about two.