＇Container＇ complexes in which a guest molecule is held mechanically within a cage-like host have been known for over a decade(.1,2). They provide a means to stabilize reactive intermediates(3) and to create new forms of stereoisomerism(4). Molecular capsules held together by hydrogen bonds are more recent; they are formed reversibly on timescales of milliseconds to hours, long enough for molecular motions(5) and even reactions(6) to be seen for the encapsulated species. Here we describe the synthesis and characterization of a hydrogen-bonded molecular capsule of nanometre dimensions, which is large enough to encapsulate two different molecules, This allows us to explore the size- and shape-selectivity of the encapsulation process : we see, for example, the exclusive formation of the hetero-guest pair when benzene and p-xylene are both added to a solution. This presumably reflects optimal occupancy of the capsule-two benzene guests leave too much empty space in the interior, and two p-xylene molecules make it too crowded.