The kinetic of the O2(1Δg)-photosensitized oxidation of the flavanone naringin (7-rhamnoglucosyl-4′,5-dihydroxyflavanone, FL) and its chalcone isomer (4′-rhamnoglucosyl-2′,6′,4-trihydroxychalcone, CH) in neutral and 1 mM NaOH ethanol solutions was studied using Rose Bengal as photosensitizer. The rate constants for the chemical quenching of O2(1Δg) by the flavonoids (kr) were determined using either UV–vis absorption spectroscopy or HPLC techniques, and for the total (physical + chemical) quenching (kt) were determined using time-resolved phosphorescence detection of O2(1Δg) at 1270 nm.A larger reactivity towards O2(1Δg) of CH than for FL in neutral ethanol solutions was observed, due to the extra conjugated double bond in CH. However, in alkaline media the reactivity of both isomers was larger than in neutral conditions. This behavior was associated to the increment of electron density by the formation of a carbanion in FL and to the presence of the extended conjugated π-system in the CH isomer by deprotonation of phenolic groups. In all cases, the formation of 4-hydroxybenzoic acid as ending product was detected by HPLC. Based on reported mechanisms of O2(1Δg)-mediated oxidation of flavonoids, the formation of 7-rhamnoglucosyl-5,4′-dihydroxyflavonol as primary photo-oxidation product was proposed.The reactivity towards O2(1Δg) of the aglycone of naringin (5,7,4′-trihydroxyflavanone or naringenin, NG) in alkaline conditions was lower than for the parent naringin, due to incapability of NG to form a carbanion species.These results are expected to have significance in biosynthesis, antioxidant properties and stability of naturally occurring flavonoids.