One approach to reducing the bandgap is the utilization of ternary and quaternary oxides, which have been receiving increased attention as light absorbers that, critically, also supply the potential needed to drive multi-electron oxidn. reactions. In this work, we report the discovery of antimony-alloyed bismuth vanadate (Sb-BiVO4). Through a combination of theor. predication and exptl. validation, we show that this novel photoanode material possesses a bandgap that linearly decreases below 2.4 eV with increasing Sb content. This work is enabled by the development of a novel two-step synthesis process that will broadly aid new materials discovery by providing synthetic access to a wide range of compositionally complex oxides.