A new compound Bi8O11(SO4), synthesised using ceramic techniques, has been studied using variable temperature X-ray powder diffraction, neutron powder diffraction and impedance spectroscopy. The material was found to undergo a phase change, on heating, from a complex structure stable at ambient temperatures to a high temperature form with tetragonal symmetry (P4(1)2(1)2; a=11.78840(4) Angstrom, c=22.7642(1) Angstrom). A gradual phase transition was determined to occur between similar to545 K and similar to600 K. High-resolution neutron powder diffraction data collected at 623 K were used to provide structural information on the high temperature phase. Disorder of the sulfate groups, presumably due to dynamic rotation, renders it impossible to locate the oxygen atoms bonded to S, but the distribution of the sulfate groups within the structure has been established. The bismuth oxide framework also displays oxygen sublattice disorder, and the material exhibits relatively high oxide ion conductivity.