The solid/gas complexation of anthracenebisresorcinol host (1) and ethyl acetate as a guest was monitored by pressure-decay, X-ray powder diffraction, and gravimetric/calorimetric thermal desorption analyses. It involves an exothermic (similar to 30 kcal/mol) phase transition and exhibits "vertical" adsorption as well as desorption "isotherms" at a threshold (equilibrium) pressure (P-th) Of the guest vapor. This is in accord with the phase rule for a two-component/three-phase/one-freedom system. The activation energies of adsorption and desorption are 2.3 and 34 kcal/mol, respectively; desorption at 25 degrees C takes weeks. The crystal structure of the ethyl acetate adduct illustrates how the flexible hydrogen-bonded network of host 1 adjusts itself to the small guest. Other polar guests tester, ketone, and alcohol) behave similarly and give rise to desorption-resistant stable adducts, except for the least bulky members of the ester and ketone guests, i.e., methyl acetate and acetone. The mechanism of phase transition is discussed in light of the lack of size effect as for the host and well-behaved guest-prebinding at P < P-th.