The stability to light of cyanidin-3-glucosyl arabinoside in DMSO and in water, in acid solutions with or without complexation with tannic acid was determined by measuring the decrease of the absorbance of the solutions. The values for the apparent first-order reaction rate constant and t(1/2) were calculated. The hypochromic effect observed in DMSO solutions when tannic acid was added as a co-pigment prevented the calculation of the complexation constant (K) for the anthocyanin-tannic acid, indicating that the predominating form in this system is the AH(+). Although loss of absorbance was considerably faster in DMSO than in water under the same conditions, tannic acid effectively retarded discoloration in both DMSO and water.512183185