The blueing effect of purplish petals during senescence in Hibiscus syriacus L. was investigated. Absorption spectra of old petals of purple flowers always produced small shoulders at 640 nm. Anthocyanin content was decreased in old petals, while flavone content was not changed. Resulting increased flavone/anthocyanin ratio enhanced the copigmentation effect. This effect was further accelerated by the increase of cell sap pH.