The structural origins of the selectivity of rubidium ion over other alkali metal ions by tribenzo-21-crown-7 is investigated from single-crystal X-ray diffraction : data for Cs(tribenzo-21-crown-7)(NO3), monoclinic, P2(1)/c, a = 9.598(2) Angstrom, b = 23.466(3) Angstrom, c = 23.973(5) Angstrom, beta = 93.31(1)degrees, V = 5390(2) Angstrom(3), Z = 8; data for [Rb(4,4'-bis-tert-butylbenzo,benzo-21-crown-7) (dioxane)(1.5)(H2O)(0.18)][Cl] . (dioxane)1.82(H2O), triclinic, P (1) over bar, a = 11.687(3) Angstrom, b = 12.800(4) Angstrom, c = 17.680(3) Angstrom, alpha = 75.31(2)degrees, beta = 80.01(2)degrees, gamma = 69.09(2)degrees, V = 2379.8(10) Angstrom(3), Z = 2; data for Na(4,4'-bis-tert-butylbenzo,benzo-21-crown-7) ReO4 . 0.5(i-PrOH), monoclinic, P2(1)/c, a = 24.300(5) Angstrom, b = 14.066(3) Angstrom, c = 22.676(5) Angstrom, beta = 108.06(3)degrees, V = 7369(3) Angstrom(3), Z = 8; data for and 4,4'-bis-tert-butylbenzo,benzo-21-crown-7, monoclinic, P2(1)/n, a = 16.427(2) Angstrom, b = 11.3675(9) Angstrom, c = 33.137(3) Angstrom, beta = 94.469(8)degrees, V = 6169.0(10) Angstrom(3), Z = 8. The structures reported here are the first reported for a tribenzo-21-crown-7, and the alkali metal ion complexes are the first reported structures of these ions with any 21-crown-7 ether. Different crown conformations are observed for each structure. Molecular mechanics calculations were performed on all conformers, and the results are related to the observed extraction selectivity for rubidium.