Abstract: Betalains are natural food colorants with high antioxidant activity. But the compounds are characterized by easiness to decondensation to achieve equilibrium of the reaction after dilution in solutions as food colorants. The aim of paper was to find molecular centers responsible for antioxidant activity by quantum chemical calculations. The calculations were performed by PM3 method in HyperChem 8.0 software. It was found that these are alpha C-H bonds to be broken to give radicals with less energy instead of phenolic OH groups commonly referred to as the most active sites of antioxidants. According to results of the calculations, the condensation reaction must not have a great impact upon colorant antioxidant activity. The assumptions made were confirmed by experimental results of the determination of antioxidant activity by Folin-Ciocalteu method with reagent are highly sensitive to reducing agents. Indeed, purified by solid-state extraction betacyanins of Beta vulgaris and of Opuntia joconostle really loosed the color but not an antioxidant activity for first two days of storage. It can be concluded that the trend of betalains for decondendation to reach the equilibrium of condensationdecondensation reaction after colorant solution dilution leads to decrease of color intensity (and betalain concentration) but only slightly affects the antioxidant activity.