The complexes Ln(2)L(3) center dot xH(2)O (L = 1,2-benzenedisulfonate; Ln = Sc ( 1), x = 14; Ln = Y( 2), x = 16; Ln = La ( 3), x = 7; Ln = Ce ( 4), x = 6; Ln = Pr ( 5), x = 10; Ln = Nd ( 6), x = 18; Ln = Sm ( 7), x = 18.25; Ln = Eu ( 8), Gd ( 9), Tb ( 10), Ho ( 11), Yb ( 12), Lu ( 13), all x = 16) were synthesised by treating Ln(2)O(3) with an aqueous solution of 1,2-benzenedisulfonic acid. Single crystal X-ray studies revealed isomorphous structures of the type, [LnL(H2O)(6)](2)[L] center dot 4H(2)O for 2, 8 - 13, i.e. Ln = Eu - Lu ( including Y) and, presumptively, intermediate Ln. These contain eight-coordinate Ln with the disulfonate ligand coordinating though one oxygen of each sulfonate group giving a seven-membered chelate ring. Complexes of the 'earlier' Ln are less systematically defined. The samarium complex, 7, is so far unique, with the structure [SmL(H2O)(6)] [SmL2(H2O)(4)] center dot 8 1/4 H2O. There is an eight-coordinate complex anion with two cisoid chelating disulfonate ligands ( again seven-membered rings), and four unidentate water molecules, and an eight-coordinate cation, which is similar to those of 2, 8 - 13. A novel lanthanum complex [H7O3][LaL2] (14) has been prepared by the treatment of La2O3 with an excess of aqueous 1,2-benzenedisulfonic acid and by treatment of 3 with perfluoroadipic acid in water. The complex anion forms polymeric chains with eight-coordinate lanthanum ions and mu-eta(2)(O, O'):eta(2)(O '', O ''')-disulfonate groups. Two oxygen atoms from each sulfonate group (O, O '' from one; O', O ''' from the other) bridge pairs of adjacent La ions. In addition O, O' (and O '', O ''') form seven-membered chelate rings with neighbouring lanthanum atoms. Two uncoordinated oxygen atoms of each dianion are hydrogen bonded to the H7O3+ ions. Another cationic lanthanum complex, [LaL(H2O)(7)] Cl center dot H2O (15), from attempts to crystallize [La2L3] center dot 7H(2)O, has a nine-coordinate La ion with a chelating disulfonate ligand and seven unidentate water molecules, and may provide some insight into the possible structures of complexes 3 - 6. In all structures, hydrogen-bonding involving water of crystallization links the cations and the anions in a supramolecular superstructure.