Abstract Thermodynamic analyses for the growth of group-III sesquioxides, including α-Al₂O₃, β-Ga₂O₃, and c-In₂O₃, by both ozone and plasma-assisted MBE were performed. In either case, under O-rich conditions, the driving force for III₂O₃ (III = Al, Ga, In) growth ( Δ P III 2 O 3 ) increased with increasing input partial pressure of the group-III metal ( P III o ), without generation of metal droplets. Conversely, under group-III-metal-rich conditions, Δ P III 2 O 3 decreased with increasing P III o and/or decreasing input partial pressure of O₃ or O. This decrease was caused by the formation of Ga₂O or In₂O during growth of β-Ga₂O₃ and c-In₂O₃. The decrease of Δ P Al 2 O 3 was smaller because the equilibrium constant of α-Al₂O₃ formation reaction was very large. Ga and In droplets formed at low temperatures (<420 °C), whereas Al droplets were formed at high temperatures (<820 °C), and the order that enabled growth at higher temperatures was c-In₂O₃ < β-Ga₂O₃ << α-Al₂O₃.