Epitaxial ε-gallium oxide (Ga 2 O 3 ) thin films incorporated with In were successfully grown by mist chemical vapour deposition (CVD) on c -plane sapphire substrates for bandgap tuning. In was successfully incorporated into epitaxial ε-(In x Ga 1−x ) 2 O 3 films at an In composition of x = 0.2 without inducing phase separation. Phase separation originated from the (400) bixbyite structure of (In x Ga 1−x ) 2 O 3 when x > 0.2. The solubility limit of In incorporated into ε-Ga 2 O 3 on sapphire substrates via mist CVD was therefore x = 0.2. Transmission electron microscopy measurements revealed that ε-(In x Ga 1−x ) 2 O 3 consisted of polycrystalline phases observed in the interface of the sapphire substrate and ε-phases located above the polycrystalline phase. The pole figure of ε-(In x Ga 1−x ) 2 O 3 thin films revealed that the epitaxial relationship between the ε-(In x Ga 1−x ) 2 O 3 thin film and the α-Al 2 O 3 substrate is (001) ε-(In x Ga 1−x ) 2 O 3 130||(0001) α-Al 2 O 3 11−20. The optical bandgap of the ε-(In x Ga 1−x ) 2 O 3 thin films was tuned from 4.5 to 5.0 eV without inducing phase separation.