The aim of the present work is to complete a preliminary study concerning the electronic band structure investigations of Na x Cu 1−x In 5 S 8 compounds with 0 Յ x Յ 1, which are expected to be formed at the Cu͑In, Ga͒Se 2 / In 2 S 3 interface. The band structure calculations demonstrate that for the compounds containing both Na and Cu, as the Cu content increases the band gap tends to decrease, and x-ray photoemission spectroscopy measurements show that this variation is mainly due to valence-band-maximum shift along the solid solution. The band gap strongly depends on the nature of the monovalent cation, and the band structure calculations demonstrate that the d electrons of copper are responsible for the shift of the valence band. In addition, it is worth noting that the Cu-containing compounds have indirect gaps.