β -ionone, a model compound of carotenoids ring structure, was investigated by FT-IR spectroscopy in a low-temperature argon matrix as well as using B3LYP/6-311+ +G(d,p) and MP2/6-311++G(d,p) quantum-chemical calcu- lations. The spectrum of matrix-isolated E

β -ionone was analyzed and attributed to six conformers of the compound. Then, matrix-isolated E

β -ionone was submitted to UV irradiation using either a broadband source (with di ff erent cuto fffi lters) or a narrowband laser/MOPO system (at various wavelengths). Upon 240 nm narrowband irradiation, the formation of both Z

retro

γ -ionone and Z

β -ionone was observed, the reactant and the photoproducts being in a photostationary equilibrium. Under these conditions, the matrix environment was found to hamper subsequent reactions of Z

retro

γ -ionone and Z

β -ionone, so that this last species could be observed directly for the fi rst time. Furthermore, the formation of Z

retro

γ -ionone was shown to occur directly via an intramolecular [1,5] H-atom shift and thereby, under the constraints imposed by the matrix con fi nement, the conformations assumed by this photoproduct were found to be strictly determined by those initially assumed by the reactant molecules. Broadband irradiation resulted in the completion of the reaction (disappearance of the reactant) and the sole observation of Z

retro

γ -ionone. These results imply that under these conditions the Z

β -ionone is unstable, very likely decaying to additional conformers of Z

retro

γ -ionone, as re fl ected in the broader bands due to this photoproduct observed in the infrared spectra of the broadband irradiated matrix.