Experimental details are given for the preparation of "softballs", large self-complementary molecules capable of assembly into pseudo-spherical capsules. Evidence is presented for their existence as hydrogen bonded dimers:in organic solvents, and binding affinities for the reversible encapsulation of smaller molecules of suitable size and shape are given. Studies at various temperatures result in calculated enthalpies and entropies of encapsulation that are positive; accordingly, the process is entropy driven. It is proposed that the hosts in their resting states contain two molecules of solvent such as benzene, and the encapsulation of a single large guest-the hostage-liberates the two solvents. The resulting increase in the number of free molecules gives rise to the increase in entropy observed for the exchange process. Experiments involving solvent mixtures are consistent with this rationale. Calculation of the capsule’s interior volume and molecular dynamics simulations support the experimental observations, and hint at unexpected phenomena dealing with the occupancy factors of these systems.