Stilbenoids are a class of phytomolecules with potential for medical applications. Some of the photochemical properties of these E/Z-photoreversible systems are known but less is available in the literature on the values of their quantum yields, especially on the effects of excitation on each photochemical efficiency. In fact, only a few cases have been studied on the systematic effect of changing the wavelength of the monochromatic excitation light on the quantum yield values of the individual photochemical steps involved in these reactions. The measurement of such an effect for pinosylvin (PN), in the present study, demonstrated that the quantum yields of both its forward and reverse photoreactions were sensitive to the wavelength of excitation. Overall an increasing trend with wavelength was observed for both isomers, with the absolute values for Z-PN being, in general, higher than those of E-PN in the region 280-330 nm. In addition, these values linearly correlated with each other throughout the latter spectral region, corresponding to the long-wavelength π,π* absorption band. Such findings strongly demonstrate that (i) both photochemical quantum yields of photoreversible PN are wavelength sensitive, (ii) the effect of excitation on their absolute values is more pronounced for Z-PN, (iii) the excitation wavelength effect is variable with the electronic absorption transition, and (iv) the variation of the quantum yields within the spectral range of a given electronic transition is dependent on the vibrational modes of the excited-state.