The stability of (+)-cyanidanol-3 in aqueous solution was investigated. Two degradation products were found at pH 1.4 by TLC. These were assumed to be epicatechin and the dimer. In the basic pH region, the main product was found to be epicatechin. It was most stable at about pH 3 and was labile in strongly acidic and basic solutions. The effect of oxygen in the head space of the container was remarkable, especially in basic solution. In strongly acidic solution, the degradation of (+)-cyanidanol-3 appeared to occur by simultaneous apparent 1st-order (epimerization) and 2nd-order (dimerization) reactions. The rate constants were calculated by a new method. In the neutral and basic pH regions, the reverse reaction between (+)-cyanidanol-3 and epicatechin took place and the rate constants were obtained.