Polymorphic phase transitions in Ba4Nb2O9 were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature alpha-modification and high-temperature gamma-modification, with the endothermic phase transition at 1176 degrees C. The alpha ->gamma transformation is accompanied by the formation of a 120 degrees domain structure, which is a consequence of hexagonal -> orthorhombic unit cell reconstruction. Reheating the presintered gamma-Ba4Nb2O9 results in the formation of a metastable gamma'-modification (formerly known as beta-polymorph) in the temperature range between 360 degrees and 585 degrees C, before the gamma ->alpha transformation at 800 degrees C. Above similar to 490 degrees C Ba4Nb2O9 becomes moderately sensitive to a loss of BaO. In air the surface of Ba4Nb2O9 grains decomposes to nanocrystalline Ba5Nb4O15 and BaO, which instantly reacts with atmospheric CO2 to form BaCO3. Surface reaction delays gamma ->alpha transformation up to 866 degrees C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba3Nb2O8 phase is observed above 1150 degrees C.