Structure and electrical conductivity of Bi 14 WO 24 as a function of temperature have been examined by X-ray and neutron powder diffraction, a.c. impedance spectroscopy and differential thermal analysis. The room temperature structure was successfully refined using a monoclinic subcell model in space group I2/m. However, additional reflections in the neutron data are consistent with a large supercell of dimensions a=17.3780(1)Å, b=17.3891(1)Å, c=26.1785(2)Å and β=90.270(1)°, as previously proposed. Transitions to tetragonal and cubic phases are observed at ca. 35°C and 780°C, respectively. The structure of the high temperature polymorph is confirmed as a fully disordered δ-Bi 2 O 3 type phase. Analysis of the defect structure is consistent with a predominantly tetrahedral environment for tungsten, as seen at low temperatures. The conductivity behaviour is correlated with the appearance of the δ-phase at high temperatures and exhibits a value of 0.97Scm −1 at 800°C.