The macrocyclic polyether 18-crown-6 forms complexes with hydrates of strong acids derived from transition metal chlorides and hydrochloric acid. The following charged-component complexes have been isolated and characterized: 18-crown-6.H3O+.FeCl4−, 18-crown-6.H3O+.InCl4−, and (18-crown-6.H3O+)2.Pd2Cl62−. X-ray diffraction study of the palladium complex is reported. The oxonium ion is located at the center of the macrocyclic ether cavity and the symmetry of the ring is close toD3d.Keywords: 18-crown-6 complexes, oxonium ions, X-ray diffraction, crystal structure