Order-disorder transitions in barium indate. Ba2In2O5 have been studied using in-situ Xray and neutron diffraction, At room temperature, the crystal structure is an orthorhombic brownmillerite structure. At 706 degreesC, the crystal structure is orthorhombic, possibly of the Imma or Ibm2 space groups. At 900 degreesC, oxygen vacancies begin to disorder. The order-disorder transition occurs slowly in two steps over a temperature range of 900 - 925 degreesC. Above this temperature range, the crystal structure is tetragonal, most likely belonging to the space group I 4/mcm. A second order-disorder transition begins at 1040 degreesC, and proceeds over the temperature range 1040-1065 degreesC. Above this temperature range, the crystal structure is a cubic, oxygen-deficient perovskite structure, with space group Pm3m. At an undetermined temperature above 1200 degreesC, Ba2In2O5 begins to decompose.