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Background Excess 5-aminolevulinic acid (ALA) and α-aminoacetone (AA) are implicated in ketosis, porphyrinpathies and diabetes. Pathologic manifestations involve O 2 − , H 2 O 2 , OH, enoyl radicals ( ALA and AA) and their oxidation end products. Methods To characterize enoyl radicals resulting from reaction of OH radicals with ALA and AA, micromolar OH concentrations were produced by pulse radiolysis of ALA and AA in aqueous solutions. Results ALA and AA react with OH at k = 1.5 × 10 9 M − 1 s − 1 . At pH 7.4, the ALA absorbance spectrum has a maximum at 330 nm (ε = 750 M − 1 cm − 1 ). This band appears as a shoulder at pH 8.3 where two ALA species are present: (NH 3 ) + -CH 2 -CO-CH 2 -CH 2 -COO − and NH 2 -CH 2 -CO-CH 2 -CH 2 -COO − (pKa = 8.3). At pH 8.3, ALA reacts with oxygen (k = 1.4 × 10 8 M − 1 s − 1 ) but not with O 2 − . At pH 8.3, AA oxidation produces two AA species characterized by an absorbance spectrum with maxima at 330 and 450 nm. ALA and AA are repaired by antioxidants (quercetin (QH), catechin, trolox, ascorbate) which are semi-oxidized (k > 10 8 M − 1 s − 1 ). QH bound to HSA or to apoferritin and ferritin repairs ALA and AA. In O 2 -saturated apoferritin solutions, Q, O 2 − , AA and reaction product(s) react with QH. Conclusions The optical absorption properties and the time evolution of ALA and AA were established for the first time. These radicals and their reaction products may be neutralized by antioxidants free in solution or bound to proteins. General significance Adjuvant antioxidant administration may be of interest in pathologies related to excess ALA or AA production. [ABSTRACT FROM AUTHOR]