Water oxidation catalysts have been usually used to enhance the charge injection efficiency of WO₃ anodes. Herein, we simultaneously improve the charge separation efficiency and charge injection efficiency, as well as the oxidation selectivity of WO₃ for photoelectrochemical water oxidation by loading CoO_x nanoparticles in a neutral electrolyte. The produced oxygen is monitored by a Clark electrode, which shows a high Faradaic efficiency on CoO_x/WO₃. In an effort to gain insight into the role of CoO_x on the high Faradaic efficiency, the rotating ring–disk electrode system was used to detect the products of the CoO_x/WO₃ heterojunction in the photoelectrochemical water oxidation process. The results show that the loading of CoO_x nanoparticles can improve oxidation selectivity of WO₃ and therefore improve the Faradaic efficiency of WO₃.