Applications, both potential and realised, for molecular capsules are manifold. Encapsulation of guest species 8 within the confined cavity spaces of these systems can result in unusual effects on properties and behaviours. Reactivity, in particular, can be altered upon encapsulation, 10 which can be exploited for both stabilisation and promotion of chemical reactions. 12 Fig. Single-crystal X-ray diffraction (SCXRD) structures of K + complexes of Pedersen's macrocyclic 18-crown-6 ether and Lehn's macrobicyclic [2.2.2]cryptand.