The present paper studies the pathways of phase transformation of heterophasic nanopowder (NP) Bi2O3 produced by pulsed electron beam evaporation (PEBE) in vacuum (4 Pa) when annealing in air. The phase transformation after annealing in air of heterophase bismuth oxide NPs leads to the formation of bismuth carbonate at an annealing temperature of 200–300 °C. The effect of NP aging in storage under normal conditions was investigated. © 2023 Elsevier Ltd and Techna Group S.r.l.