The photochemical behaviour of β-ionone, β-ionylidene acetaldehyde, β-ionylidene acetonitrile and ethyl-β-ionylidene acetate has been investigated in β-cyclodextrin as the host. They all form a fairly stable and water-soluble complex with β-cyclodextrin. An impressive difference in the behaviour of β-ionone and β-ionylidene acetaldehyde was observed between solution (in benzene and methanol) and as a cyclodextrin complex (in water). While in organic solvents β-ionone gives rise to products arising from geometric isomerization and 1,5-hydrogen migration, in cyclodextrin only 1,5-hydrogen migration occurs. Similarly, while in solution β-ionylidene acetaldehyde gives only geometric isomers as the final products; in cyclodextrin it gives retro-γ product derived via 1,5-hydrogen migration as the final product. No difference in the behaviour of β-ionylidene acetonitrile and ethyl β-ionylidene acetate was observed between solution and cyclodextrin complex. The variation in the photochemical behaviour of these compounds has been rationalized on the basis of the effect of cyclodextrin on the nπ*-ππ* character of the lowest excited state.