Contributions into tryptophan ionization from non-relaxed prefluorescent S* and relaxed fluorescent S, states are separated by measurement of the ionization quantum yield in neutral and extremely acidic solutions. The rate constant of the ionization from the prefluorescent S* state is temperature independent, the rate constant of ionization from S, state has frequency factor A(ion) = (2.3 +/- 0.9) x 10(15) s(-1) and activation energy E-ion = 50 +/- 5 W/mol. For L-tryptophan at room temperature the ionization from S, state is completely suppressed by other processes of S, state decay, in particular - by the intramolecular proton transfer (IPT) from the protonated amino group to the indole ring (A(ipt) = (6.1 +/- 1.9) x 10(10) s(-1), E-ipt = 14.0 +/- 1.8 kJ/mol). (C) 2004 Elsevier B.V. All rights reserved.