The reaction of (+)-cyanidanol-3 with sodium sulfite in aqueous solution was investigated. (+)-Cyanidanol-3 was degraded by sodium sulfite in aqueous solution to afford a water-soluble degradation product, which was assumed to be (+)-cyanidanol-3 carrying the sodium sulfonate function in place of the aliphatic hydroxy group at the C-3 position. Further, it was found that the rate of degradation of (+)-cyanidanol-3 by sodium sulfite was approximately the same as that of epicatechin, that the rate was proportional to the concentration of sodium sulfite and to the sum of the concentrations of (+)-cyanidanol-3 and sodium sulfite, and that (+)-cyanidanol-3 or epicatechin was degraded more rapidly by sodium sulfite as the pH was increased. The activation energy of the degradation was calculated to be 28 kcal/mol at pH 8.0 from Arrhenius plots.