High-temperature face-centered cubic bismuth oxide phase is a material of great interest given its unique properties. In the present study, ?-Bi2O3 and tantalum powders were used as the starting powders for the formation of high-temperature bismuth oxide phase via mechanochemical synthesis by high energy ball milling. (Bi2O3)80(Ta)20 and (Bi2O3)95(Ta)5 in weight concentrations were milled in either an oxygen-free argon-filled glove box environment or an ambient atmosphere to investigate the effects of oxygen concentration and tantalum addition. The as-milled powders were examined using X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, and differential scanning calorimetry to reveal the structural evolution. The experimental results showed that for (Bi2O3)95(Ta)5 powder mixtures milled within the glove box, tantalum gradually reacted with the ?-Bi2O3 phase and formed a ?-Bi7.8Ta0.2O12.2 phase. For (Bi2O3)80(Ta)20 milled under the same conditions, Ta and ?-Bi2O3 mechanochemically reacted to form ?-Bi3TaO7 and bismuth after 10 min of high energy ball milling, whereas milling (Bi2O3)80(Ta)20 under the ambient atmosphere with a much higher oxygen concentration accelerated the mechanochemical reaction to less than five minutes of milling and resulted in the formation of high-temperature ?-Bi3TaO7 phase.