THE encapsulation of molecules within larger molecular or supramolecular cages (see ref. 1 for examples) brings about many interesting phenomena, among them the stabilization of reactive intermediates(2), new forms of stereoisomerism(3), and templating of the cage itself 4,5. Here we describe a self-assembling hydrogen-bonded molecular capsule that encapsulates guest molecules in a reversible, apparently entropy-driven process, giving rise to unusual temperature dependence of the formation process. The positive entropy of formation-at first sight unexpected-seems to be the result of the fact that more than one solvent molecule is included in the ’bare’ cage, making their replacement by a single large molecule (here adamantane or ferrocene) entropically favourable.