CYCLODEXTRINS and modified derivatives can bind, and sometimes modifg the properties of, guest molecules in their torus-shaped cavities(1,2). They have also been used as the building blocks of molecular materials and devices(3). The propensity to bind and retain a guest is not easily predictable or controllable, however. There is currently much interest in the switching on and off of chemical(4) and biological(5) activity, particularly by photochemical means(6), as such functions will be required of molecular-scale devices. Here me report the controlled binding and release of guest molecules in cyclodextrins modified with substituents that can reversibly form bridging units across the cavity openings. Irradiation of percinnamoylated alpha- or beta-cyclodextrin in N-methylpyrrolidin-2-one (NMP) leads to the formation of intramolecular cyclobutane bridges which trap a bound NMP molecule. Irradiation at a different wavelength breaks open the cyclobutane rings and releases the guest.