The E/Z photo-isomerization behaviour of the cinnamoyl chromophore in some cyclic polyamine alkaloids was investigated. Under daylight laboratory conditions, the alkaloid iso-18-O-methylchaenorpine (3) with an E-double bond isomerises gradually to the Z-form 18-O-methylchaenorpine (1). This can be enhanced by irradiation with visible light. On the other hand, the isomer 18-O-methylchaenorpine with a Z-double bond, photoisomerises to the E-form in 70% yield when irradiated with a low pressure mercury lamp at 253.7 nm. This behaviour is also observed in the solid phase. Thus two new alkaloids, isochaenorpine (4) and isochaenorbine (6), were produced by the photo conversion of their Z-forms. © 1990.