Reaction of MCl3. nH(2)O (M = La, Eu) with 18-crown-6 in HCl/toluene affords the crystalline oxonium complexes (H9O4)[LaCl2(H2O)(18-crown-6)]Cl-2 (1) and (H3O)[EuCl(H2O)(2)(18-crown-6)]Cl-2 (2). In complex 1 the H9O4+ cation is of the unusual linear chain type with O ... O contacts of 2.560(9), 2.430(9), and 2.483(9) a. The analogous reaction with YbCl3. 6H(2)O results in the isolation of the known species (H5O2)[H3O .(18-crown-6)]Cl-2. Analogous reaction of YbCl3. 6H(2)O with 15-crown-5 gives [Yb(H2O)(8)]Cl-3.(15-crown-5) (5). Reaction of uranyl acetate with 15-crown-5 within a toluene/HCl mixture results in the formation of a yellow liquid clathrate layer from which crystals of (H5O2)[UO2(H2O)(2)Cl-3]. 2(15-crown-5) (6) are deposited. The H5O2+ ion in this material exhibits a particularly short O ... O separation of 2.371(8) A. On standing, loss of HCl results in the transformation of 6 into a species of empirical formula [UO2(H2O)(3)Cl-2].(15-crown-5) (7) which has been shown by X-ray crystallography to contain 16 unique uranium complexes, arranged in infinite hydrogen-bonded chains in the solid state. Crystal data for 1 : monoclinic, P2(1)/c, a = 7.6950(2) Angstrom, b = 21.2160(16) Angstrom, c = 15.8650(10) Angstrom, beta = 92.866(2)degrees, V = 2586.8(3) Angstrom(3), Z = 4. 2 : orthorhombic, Pbn2(1), a = 10.2267(6) Angstrom, b = 14.7231(9) Angstrom, c = 29.8565(18) Angstrom, V = 4495.5(5) Angstrom(3), Z = 4. 5 : monoclinic, P2(1)/n, a = 9.1740(3) Angstrom, b = 17.1900(3) Angstrom, c = 15.2450(5) Angstrom, beta = 92.643(2)degrees, V = 2401.60(12) Angstrom(3), Z = 4. 6 : orthorhombic, Pmcn, a = 12.1740(5) , b = 14.7740(7) Angstrom, c = 18.4920(11) Angstrom, V = 3325.9(3) Angstrom(3), Z = 4. 7 : trigonal, P3(2), a = 35.3500(2) Angstrom, c = 21.3755(2) Angstrom, V = 23 132.7(3) Angstrom(3), Z = 48.