The evolution of the band structure of β-In2S3-3xO3x (BISO) thin films grown by physical vapor deposition, with composition x, is investigated using x-ray photoelectron spectroscopy. It is shown that the energy difference between the valence-band level and the Fermi level remains nearly constant as the optical band gap of the films increases. As a consequence, the difference between the conduction band level and the Fermi level increases as much as the optical band gap of the films. The calculation of the electronic affinity χ of the BISO thin films shows that it decreases linearly from 4.65 to 3.85 eV when x varies from 0 to 0.14. This will facilitate fabrication of efficient Cu(InGa)Se2-based solar cells having different absorber layer band gap. © 2003 American Institute of Physics.