The difference between fluorescence lifetimes of the following amino acids: phenylalanine (Phe), tyrosine (Tyr), (O-methyl)tyrosine (Tyr(Me)), (3-hydroxy)tyrosine (Dopa), (3,4-dimethoxy)phenylalanine (Dopa(Me)(2)) and their amides was used to testify the mechanism of fluorescence quenching of aromatic amino acids by the amide group. On the basis of the Marcus theory of photoinduced electron transfer parabolic relationships between In k(ET) and ionization potentials reduced by energy of excitation (IP - E-0.0*) for the above-mentioned amino acids were obtained. This finding indicates the occurrence of photoinduced electron transfer from the excited chromophore group to the amide group.