We proposed the nitrone–oxaziridine rearrangement as a novel method for photochemical activation for the homolysis of alkoxyamine in nitroxide-mediated polymerization. The photoisomerization of the aldo-/ketonitrone-group into the oxaziridine one in 2,5-dihydroimidazole 3-oxide-based alkoxyamines was studied; the products of photolysis have been identified, and quantum yields were measured. Conversion