Abstract High oxide-ion conducting material δ-Bi 2O 3 was electrodeposited on stainless steel and polycrystalline Au substrates at low temperature. Both X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) images revealed nanocrystallites about 100 nm in size inside the electrodeposited δ-Bi 2O 3 thin films. High-resolution TEM images showed the existence of smaller nanocrystallites 10–20 nm in diameter. The long-term stability of the electrodeposited δ-Bi 2O 3 was checked by XRD measurements on 1-year-old samples. Its thermal stability was studied through a series of annealing at different temperatures. At an annealing temperature of 340 °C, the δ-Bi 2O 3 fcc structure begins to change towards that of Sillenite ( bcc).