Photolysis of aqueous kynurenine (KN) solutions results in the formation of triplet kynurenine (KN)-K-T*. In low pH solutions, triplet formation occurs with almost 100% efficiency, while in neutral solutions the triplet quantum yield is Phi(T) = 0.018 +/- 0.004. The dissociation constant of (KN)-K-T*, which is attributed to deprotonation of the anilino group, has a pK(a), value of 4.7. Similar triplet absorption spectra were obtained under direct and acetone-sensitized photolysis. The large difference in quantum yields as a function of pH is attributed to excited-state properties of the first excited singlet state of KN. The rate constant quenching for (KN)-K-T* by oxygen is k(q) = 2 x 10(9) M-1 s(-1).