Earth-abundant materials are required to facilitate upscaling of renewable hydrogen generation. Here, the synthesis of a novel oxidic ternary cocatalyst containing molybdenum, chromium, and copper, which has been found to be highly active in the overall photocatalytic splitting of water over gallium oxide, is described. With the noble metal-free system, requiring hydrogen evolution rates comparable to that of the well-established RhxCr2?xO3/Ga2O3 water splitting cocatalyst is achieved. Although the stability of the as-prepared ternary cocatalyst system appeared to be poor, the cocatalyst can be easily regenerated and stabilized by an oxygen treatment under ambient conditions. Furthermore, higher MoOx loadings were found to be more active and showed improved stability. By means of careful characterization using X-ray-based spectroscopy and TEM, the function of the individual cocatalyst compounds was closely examined, suggesting synergetic interactions of molybdena and chromia stabilizing CuO against pho