Abstract: Singlet oxygen was generated by means of rose bengal under irradiation by visible light. N1-acetyl-5-methoxykynuramine (AMK) was rapidly destroyed by this reactive oxygen species, whereas its formylated precursor, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK), was remarkably inert. At photon fluence rates of 1400 μmol photons/m2s, and using 20 μm rose bengal, most of initially 0.2 mm AMK was destroyed within 2 min, whereas AFMK remained practically unchanged for much longer periods of time. Competition experiments with other scavengers revealed the following order of reactivity towards singlet oxygen: diazabicyclo-[2,2,2]-octane (DABCO) << imidazole < 4-ethylphenol < Nα-acetylhistidine < histidine < melatonin < AMK, the last one being about 150 times more effective than DABCO. Contrary to the oxidation in free radical-generating systems, AMK did not form adducts with the tyrosine side chain fragment, 4-ethylphenol, under the influence of singlet oxygen. In UV-exposed cells (keratinocytes, plant cells) it is likely to be more rapidly destroyed by singlet oxygen than formed from AFMK.