The purpose of this work was to explore the potential of CuGa(S,Se) 2 thin films as wide-E G top cell absorbers for photoelectrochemical (PEC) water splitting. A synthesis was developed on fluorinated tin oxide (FTO) photocathodes by converting copper-rich co-evaporated CuGaSe 2 into CuGa(S,Se) 2 via a post-deposition annealing. We found it necessary to first anneal CuGaSe 2 at lowtemperature in sulfur then at high-temperature in nitrogen to preserve the transparency and conductivity of the FTO. Using this two-step synthesis, we fabricated a 1.72 eV CuGa(S,Se) 2 photocathode with a saturation current density and photocurrent onset potential of 10 mA/cm 2 and −0.20 V versus reversible hydrogen electrode, respectively. However we found that the PEC performance and sub-E G transmittance, worsened with increasing copper content. Using flatband potential measurements and the Gerischer model, we show that divergences in PEC performance of CuGa(S,Se) 2 photocathodes can be explained by differences in conduction band minimums and Fermi levels. We also explain that sub-E G transmittance is likely hampered by a defect band 100−400 meV below E C . Additional external quantum efficiency measurements of a high-efficiency 1.1 eV Cu(In,Ga)Se 2 photovoltaic driver, while shaded by the CuGa(S,Se) 2 photocathode, yielded a short-circuit current density of 4.14 mA/cm 2 revealing that CuGa(S,Se) 2 shows promise as a top cell for PEC water splitting.