It was reported that the acyl moieties of acylated anthocyanins with hydroxycinnamic acids were photoisomerized from the trans-to the cis-isomer by light irradiation. We studied the effects of light irradiation on nasunin, p-coumaroylated anthocyanin from eggplant, and characterized the photo-isomerized anthocyanin by high performance liquid chromatography and fast atom bombardment mass spectroscopy. The photo-isomerization occurred under acidic conditions and was repressed with c!-glucosylrutin that strongly absorbed ultraviolet light.