The photochemical behaviour of stilbenes and alkyl cinnamates complexed to cyclodextrin has been investigated in aqueous media. In benzene, on direct excitation, the photostationary state of stilbene, p-methylstilbene, p-methoxystilbene and p-fluorostilbene contains predominantly the cis isomer (greater than 85%); excitation of cyclodextrin complexes of either cis- or trans-stilbene in aqueous media resulted in a photostationary state enriched in trans isomers. Furthermore, the cyclization product of cis-stilbene, i.e. phenanthrene, which is formed in detectable amounts in benzene solution was found to be absent during aqueous β-cyclodextrin irradiation. However, for cinnamate esters the behaviours in solution and in cyclodextrin are identical. Irradiation in organic solvents such as benzene and methanol and irradiation in aqueous β-cyclodextrin solution result in the same 1:1 mixture of cis and trans isomers. The above results have been rationalized on the basis of the influence of the cyclodextrin cavity on the decay ratio of the twisted olefins.