Nb 4 W 1 3 O 4 7 , a mixed valency oxide crystallizing in the structure of the 4:9 phase (4Nb 2 O 5 .9WO 3 ), was oxidized at various temperatures. The products have been characterized by high-resolution transmission electron microscopy. Oxidation products obtained at 600 and 1000 o C exhibit the development of planar defects running along {110} of the TTB subcell. The defects are built up by structural elements observed here for the first time: Two or more five-membered rings of octahedra are connected via two common octahedra. These arrays are combined with blocks of ReO 3 -type structure and ordered along the defect planes. The oxidation at 1200 o C leads to a separation into the 2:7 phase and WO 3 .