Abstract: Melatonin had previously been shown to reduce up to four 2,2‘-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) cation radicals (ABTS•+) via a scavenger cascade ending with N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK). However, when melatonin is added to the reaction system in much lower quantities than ABTS•+, the number of radicals scavenged per melatonin molecule is considerably higher and can attain a value of ten. Under conditions allowing for such a stoichiometry, novel products have been detected which derive from AFMK (1). These were separated by repeated chromatography and the major compounds were characterized by spectroscopic methods, such as mass spectrometry (HPLC-MS, EI-MS and ESI-HRMS), 1H nuclear magnetic resonance (NMR) and 13C NMR, heteronuclear multiple bond connectivity (HMBC) correlations. The identified substances are formed by re-cyclization and represent 3-indolinones carrying the side chain at C2; the N-formyl group can be maintained, but deformylated analogs seem to be also generated, according to MS. The primary product from AFMK (1) is N-(1-formyl-5-methoxy-3-oxo-2,3-dihydro-1H-indol-2-ylidenemethyl)-acetamide (2), which is obtained after purification as E- and Z-isomers (2a, 2b); a secondary product has been identified as N-(1-formyl-2-hydroxy-5-methoxy-3-oxo-2,3-dihydro-1H-indol-2-ylmethyl)-acetamide (3). When H2O2 is added to the ABTS•+ reaction mixture in quantities not already leading to substantial reduction of this radical, compound 3 is isolated as the major product, whereas 2a and 2b are virtually absent. The substances formed differ from all previously known oxidation products which derive from melatonin and are, among these, the first 3-indolinones. Moreover, the aliphatic side chain at C2 is reminiscent of other substances which have been synthesized in the search for melatonin receptor ligands.