The ground- and excited-state protonation of Coumarin 102 (C102), a fluorescent probe applied frequently in heterogeneous systems with an aqueous phase, has been studied in aqueous solutions by spectroscopic experiments and theoretical calculations. For the dissociation constant of the protonated form in the ground state, pK(a) = 1.61 was obtained from the absorption spectra; for the excited-state dissociation constant, pK(a)* = 2.19 was obtained from the fluorescence spectra. These values were closely reproduced by theoretical calculations via a thermodynamic cycle (the value of pK(a)* also by calculations via the Forster cycle) using an implicit explicit solvation model (polarized continuum model + addition of a solvent molecule). The theoretical calculations indicated that (i) in the ground state, C102 occurs primarily as a hydrogen-bonded water complex, with the oxo group as the binding site, (ii) this hydrogen bond becomes stronger upon excitation, and (iii) in the ground state, the amino nitrogen atom is the protonation site, and in the excited state, the carboxy oxygen atom is the protonation site. A comprehensive analysis of fluorescence decay data yielded the values k(pr) = 3.27 x 10(10) M-1 s(-1) for the rate constant of the excited-state protonation and k(dpr) = 2.78 x 10(8) s(-1) for the rate constant of the reverse process (k(pr) and k(dpr) were treated as independent parameters). This, considering the relatively long fluorescence lifetimes of neutral C102 (6.02 ns) and its protonated form (3.06 ns) in aqueous media, means that a quasiequilibrium state of excited-state proton transfer is reached in strongly acidic solutions.