Photoexcitation of 1-thiouredopyrene-3,6,8-trisulfonic adducts (TUPS) of amino acids by the third harmonic frequency of a Nd:YAG laser (355 nm) generates the triplet state of the dye with high quantum efficiency. Relaxation of the triplet proceeds in anaerobiosis with a half decay time of 0.5 ms. The relaxation rate increases 100-fold in the presence of dioxygen. A radiative transition between the triplet and the ground state of the dye results in phosphorescent emission centered at 658 nm. The excited state of TUPS, being a strong reductant, can donate its electron to a variety of acceptors. Transient absorption spectroscopy was used to directly measure the photoinduced electron transfer from the excited dye to rhodamine B (RB) and cytochrome c. The reaction with RB was followed by monitoring the oxidation of the triplet state of TUPS at 487 nm (epsilon = 25 000 +/- 5 000 M-1 cm-1) or the reduction of RB at 553 nm. The second order rate constant for the reaction was found to be (2.5 +/- 0.2) x 10(9) M-1 s-1, a value compatible with that for diffusion controlled reactions. When directed to cytochrome c the photoinduced perturbation causes rapid reduction of the protein's heme group, seen as a monophasic increase of absorbance at 550 nm. The combination of appropriate redox properties with the capability of covalent protein modification makes the dye useful for initiation and analysis of electron transfer reactions in chemical and biological systems.