The crystal of p-tert-butylthiacalixarene (2) includes mono-, di-, and trimethylamines from their respective aqueous solutions to give 1:1 (host/guest) inclusion crystals. In competitive inclusion of these three amines, dimethylamine is selectively included. X-ray crystallographic analysis of the inclusion crystals of dimethylamine and trimethylamine reveals that the amines are included by forming salts with the calixarene (H4L). In the inclusion crystal of dimethylamine, two dimethylammonium ions and two water molecules form an aggregate, which is interposed between two calix anions (H3L-) arranged in a tail-to-tail manner to construct an exo complex. In the inclusion crystal of trimethylamine, however, a trimethylammonium ion is included in the cavity of the calix anion to form an endo complex. In competitive inclusion of dimethylamine and trimethylamine, the guest selectivity can be switched by changing the polarity of solvent; dimethylamine and trimethylamine are selectively included from low- (32.4 ≤ ϵ ≤ 65.9) and high-permittivity solvents (88.9 ≤ ϵ ≤ 132.6), respectively. Mechanistic studies reveal that the inclusions of dimethylamine and trimethylamine are favored under kinetic and thermodynamic control, respectively.