The rate of excited-state proton transfer from four cyano-substituted 2-naphthols was measured in H2O and D2O by picosecond time-resolved fluorescence. The excited-state acidity constant, pK(a)*, was calculated and compared with previous results and was found to be substitution-dependent, as predicted by the charge-transfer theory. The isotope effect was evaluated for all the compounds of the series. As reported for many other hydroxyaromatic compounds, this effect was more important for the dissociation rate constant than for the recombination rate constant.