Abstract This bright yellow salt, a component of certain mixtures that have been crystallized from solutions of hydrolysed uranyl nitrate, has been isolated as the fraction which is insoluble in acetone. The dimensions of its triclinic unit cell have been derived from X-ray data. The composition and properties of the salt are interpreted in terms of it being the nitrate of a large +2 cation containing a hexagonal ring of six uranyl groups joined by twelve diol bridges, and on this basis the formula assigned to the salt is [(UO 2) 6(OH) 12(H 2O) 12H 2](NO 3) 2· xH 2O, where x is ca. 6. The components of the mixture formed by loss of water at 70° have been examined, and the disproportionation reaction that occurs slowly in aqueous solution has been studied.