The electronic structure of photocatalyst AgInS2 has been investigated by first-principles calculation. Our results demonstrate that the band edge positions of chalcopyrite and orthorhombic AgInS2 straddle the water redox potentials. From the band offset calculation, we find that Ga-doping of AgInS2 shifts the conduction band minimum upwards, whereas Cu-doping of AgInS2 shifts the valence band maximum upwards. By (Ag,Cu)(In,Ga)S-2 alloying one can thereby tailor both the band edge positions and the band gap energy, and this effect provides an approach to optimize the band properties for overall water splitting. (C) 2013 Elsevier B.V. All rights reserved.