The structure of potassium yttrium hexaniobium octadecachloride is built of anionic [Nb(6)Cl(12)(i)Cl(6)(a)](4-) cluster units (where 'i' and 'a' denote inner and outer ligands, respectively), linked together by K(+) and Y(3+) cations. The K(+) cations occupy half of the tetrahedral vacancies in the face-centered cubic lattice of cluster units, and are coordinated by 12 chloride ligands. The Y atom is located in an octahedral site and is bonded to six outer chloride ligands.