Amorphous gallium indium zinc oxide (a-GIZO) thin films of different compositions (Ga2O3:In2O3:ZnO=1:1:1,2:2:1,3:2:1,4:2:1) on Si substrates were investigated by high-resolution x-ray photoelectron spectroscopy and x-ray absorption spectroscopy (XAS) using synchrotron radiation. The O 1s, Ga 3d, In 4d, Zn 3d core, and shallow-core levels as well as the valence band maxima and O K-edge XAS were investigated. Each O 1s spectrum could be deconvoluted by a main component (O1 in the text) representing the Ga–In–Zn–O quaternary system along with two other higher-binding energy (BE) components (O2 and O3 in the text). The O2+O3 intensity increased as the Ga2O3 content increased. For the as-prepared samples, the spectral peak separations between the Ga 3d (∼20 eV) and Zn 3d (∼11 eV) orbitals and between the In 4d (∼18 eV) and Zn 3d orbitals became larger, respectively, as the Ga2O3 content increased. For the surface-cleaned samples, this trend was the same but with smaller increases in their separations. The sputter-cleaning effectively reduced the Zn 3d intensity by ∼30% relative to those of Ga 3d and In 4d. The valence band maximum shifted toward higher BE, up to ∼0.5 eV for the as-prepared samples and ∼0.25 eV for the cleaned samples, and the conduction band minimum (measured at the O K-edge) was measured at photon energies ranging upwards to ∼0.2 eV as the Ga2O3 content increased, demonstrating that the band gap can be tailored by increasing the Ga2O3 content. The effects of increasing Ga2O3 contents on the local chemical states and the corresponding electrical conduction are discussed in this paper.