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Abstract— Novel synthetic polyene polyketones and new synthetic capsorubin isomers were examined for their ability to quench singlet molecular oxygen (1O2) generated by the thermodissociation of the endoperoxide of 3,3'-(l,4-naphthylene) dipropionate (NDPO2). C28-polyene-tetrone (1) exhibits the highest physical quenching rate constant with 1O2 (kq= 16 × 109M-1 s-1). For comparison, the rate constant for the most efficient biological carotenoid, lycopene (3) is kq= 9 × 109M-1 s-1 that of β-carotene (5) kq= 5 × 109M-1 s-1. The presence of two oxalyl chromophores at the ends of the polyene chain seems to enhance the 1O2 quenching ability in the C28-polyene-tetrone (1). C28-polyene-tetrone-diacetal (2) (kq= 9 × 109M-1 s-1) and C40-epiisocapsorubin (4) (kq= 8 × 109M-1 s-1) also have high 1O2 quenching abilities. Two carotenoids from plants, phytoene and phytofluene, were much less efficient, kq values being below 107M-1 s-1. Due to the very high singlet oxygen quenching abilities, C28-polyene-tetrone (1), C28-polyene-tetrone-diacetal (2) and C40-epiisocapsorubin (4) may have potential use in preventing 1O2-induced damage in biological and non-biological systems.