This paper reports the crystal and molecular structures of a macrocyclic polyether ligand, 2,3:4,5-bis[l,2-(3-methyl-naphtho)]-l,6,9,12,15,18-hexaoxacycloeicosa-2,4-diene (C32H36O6, I), and of its 1:1 complex with ;m-butylammonium perchlorate (II), as determined by X-ray diffraction methods. The inclusion complex crystallizes with 1 mol of benzene in the triclinic space group P with Å = 8.902 (5) Å, b = 11.117 (5) Å, c = 20.885 (12) Å, a = 91.69 (4)°,β = 91.07 (5)°,y = 96.30 (4)°, and Z = 2. Crystal data of the uncomplexed hexaether: a = 8.738(3)A, b = 12.037 (5) Å, c = 13.771 (4) Å, a = 104.11 (3)°, (3 = 84.57 (3)°, y = 96.46°, Z = 2, space group P1. The host molecules are conformationally disordered in the crystal when uncomplexed, but have an ordered structure in the complex they form with (CH3)3CNH3+C104_. The observed geometry of the intermolecular host-guest type association is correlated with that found in previous studies of related compounds. Structural data confirm that two types of interactions binding an ammonium guest to a macrocyclic polyether host are important: (a) +N H-O hydrogen bonds and (b) direct N+-O pole-dipole attractions where one of the lone-pair orbitals of a donor oxygen is directed at the electrophilic N. Inspection of the molecular structures reveals that the preferred overall conformation of this ligand is asymmetric, the mean plane of the macroring forming an angle of about 40° with the 1,1 ‘-dinaphthyl bond and approaching one of the methyl substituents. As a result, the two faces of the macrocyclic cavity are equivalent with respect to the complexation of an ammonium guest only by virtue of rapidly established equilibria in solution between conformers.