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, kPT ≈ 1 × 10 13 s-1, the fastest rate reported up to now. On the basis of the Förster-cycle, the pKa* 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 ∼1.25 × 1012 s-1, one-eighth that of the QCy9 compound. We found a kinetic isotope effect of the ESPT of about two.