The global analysis of the fluorescence intensity decays of tyrosine across the emission spectrum at pH = 0 revealed that its fluorescence intensity decay is bi-exponential with fluorescence lifetimes equal to 63 +/- 7 ps and 1.030 +/- 0.009 ns, whereas, the global analysis of the fluorescence intensity decays of tyrosine at different pH revealed that the quenching of tyrosine fluorescence by H3O+ ions is connected with the presence of protonated carboxyl group. The lack of acid-base equilibrium in the excited state or disturbance of this equilibrium by an additional quenching process caused the discrepancy between experimental points and Hendersson-Haselbalch relationship. (C) 2002 Elsevier Science B.V. All rights reserved.