The primary particle (or crystallite) size effect on the order–disorder phase transition temperature in Ba2In2O5 was investigated. Sintered specimens were prepared by solid state synthesis. Structural characterization performed by Raman spectroscopy shows that single-phase brownmillerite structure was obtained at room temperature for all prepared samples. In situ high-temperature X-ray diffraction experiments using Synchrotron radiation and electrical conductivity measurements reveal a decrease in the phase transition temperature with decreasing of the primary particle size. Lattice defects, mainly hydroxyls and nitrates, also influence the phase transition in Ba2In2O5.