The interior of cage-like molecules can be considered to provide a new phase of matter, in which it becomes possible to stabilize reactive intermediates and to observe new forms of stereoisomerism. Cage-like molecular complexes that self-assemble through weak intermolecular forces are dynamic species, encapsulating guest molecules reversibly. They can persist over timescales ranging from microseconds to hours, long enough for chemical processes to take place within them. Here we report the acceleration of a Diels-Alder reaction by encapsulation of the reactants in a self-assembling molecular capsule. Although product inhibition (lack of dissociation) prevents the system from showing true catalytic behaviour, there is clear evidence for a rate increase of over two orders of magnitude owing to the effective enhancement of concentration inside the capsule.