Abstract—The UV photolysis of tryptophan (Trp) and Trp‐containing peptides in aerated aqueous solutions has been studied by ESR and spin‐trapping techniques using f‐nitrosobutane as the spin‐trap. The photolysis of Trp alone at 290 nm gave rise to the addition of the spin‐trap to carbon 3 of the indole ring. A large ESR signal from the hydronitroxide spin‐adduct was also observed revealing the formation of hydrated electrons. Generally, the photolysis of Trp‐containing dipeptides generated the deamination radical of the N‐terminal amino acid followed by addition to the spin‐trap. In the case of lysyl‐Trp, a deamination radical from the side chain of lysine was proposed. A sensitization experiment with Trp as sensitizer and glycine (Gly) as substrate led to the generation of the deamination radical of Gly. Most of the observed free radicals resulting from the photolysis of Trp‐containing peptides can be explained in terms of hydrated electrons reacting with the carbonyl group followed by deamin