The correlation between the crystal structure and valence band structure of a (Ga x In 1−x ) 2 O 3 solid solution system was investigated by using combinatorial synthesis. At a low Ga content of (Ga x In 1−x ) 2 O 3 with a single-phase cubic In 2 O 3 crystal structure, a surface electron accumulation layer (SEAL), which is an important electrical phenomenon in In 2 O 3 , was confirmed. When the Ga content increased to approximately x = 0.4, mixed crystal structures of Ga 2 O 3 and In 2 O 3 were produced. Above x = 0.5, the dominant valence band structure was attributed to Ga 2 O 3 , the SEAL disappeared, and the sheet resistance increased greatly by 5 orders of magnitude or more. The in-gap state and valence band structure of the (Ga x In 1−x ) 2 O 3 solid solution system were strongly affected by Ga 2 O 3 ; however, the valence band maximum position shifted to a higher binding energy.