Abstract—The UV‐photolysis of peptides containing tyrosine (Tyr) was investigated in aqueous solutions at room temperature at 220 and 265 nm. The short‐lived free radicals formed during photolysis were spin‐trapped by t‐nitrosobutane and identified by electron spin resonance. For N‐acetyl‐ and N‐formyl‐L‐Tyr and for peptides containing L‐Tyr as the middle residue, photolysis at 265 nm under neutral conditions produced mainly spin‐adducts due to the scission between the alpha carbon and the methylene group attached to the aromatic ring, while at 220 nm decarboxylation radicals were spin‐trapped. Photolysis of di‐ and tripeptides at 275 nm in alkaline solutions predominantly generated deamination radicals. The radicals produced in the photolysis of the oxidized A chain of insulin were tentatively characterized by comparison with the resu