The stoichiometric photodecomposition of water into H 2 and O 2 proceeds over ZrO 2 powder suspended in aqueous solutions in the absence of loaded metals such as Pt, RuO 2 and NiO x . The activity is affected significantly by the pressure of the reaction system, the types of additive and the pH of the solution. Specifically, the addition of carbonate, such as NaHCO 3 and Na 2CO 3, leads to a remarkable increase in the activity and stability of the gas evolution rate vs. the reactor pressure. The surface of ZrO 2 was covered with carbonate species by the addition of carbonate salt. It is thought that carbonate species on ZrO 2 play an important role in the desorption of O 2 via the carbonate radical. In contrast, the promotion effect of platinum loading on water splitting is negligible. This behaviour can be explained in terms of the backward reaction (H 2O formation from H 2 and O 2) on Pt and the barrier height of the semiconductor-metal junction at which electrons in the conduction band migrate. Furthermore, it is concluded that the suppression of the backward reaction on Pt is essential for overall water splitting into H 2 and O 2, and this is one of the important functions of NaHCO 3 and Na 2CO 3. Pt on ZrO 2 in carbonate solutions exists as an oxide; therefore the effect of the backward reaction is suppressed.