The structure of strontium niobium oxide, Sr 5 Nb 5 O 17 , prepared by the floating-zone melting technique, has been determined by single-crystal X-ray analysis. The structure contains distorted perovskite-type slabs parallel to (100) formed from five NbO 6 octahedra and Sr atoms. The Nb-O distances range from 1.89(2) to 2.10(2)A for Nb1, from 1.809(5) to 2.282(6)A for Nb2, and from 1.851(6) to 2.167(6)A for Nb3. The NbO 6 conformations are (4+1+1) distorted. Sr2 and Sr3 are coordinated by 12 O atoms with Sr-O distances in the range 2.64 (1) to 3.04 (1)A for Sr2 and 2.50(1) to 3.06(1)A for Sr3. Sr1, placed near the boundary of the slab, is irregularly coordinated by five O atoms in the same slab and two O atoms in the neighbouring slab [Sr-O 2.46(1) to 2.915(6) A] and by a further three O atoms at long coordination distances [3.345 (8) to 3.544 (9)A]. The title compound is isotypic with La 5 Ti 5 O 17 and both structures belong to the homologous series A n+1 B n+1 O 3n+5 with n = 4, being diamagnetic (B = Nb) or paramagnetic (B = Ti) electrically conducting compounds.