Niobium oxynitride (NbON) is a promising photoanode material for photoelectrochemical water splitting using visible light up to similar to 600 rim in wavelength. However, the NH3 nitridation method widely used for oxynitride synthesis cannot convert Nb2O5 (Nb5+) to crystalline NbON (Nb5+), due to the generation of NbOxNy (Nb2+-3+). To prevent Nb reduction during nitridation, we added O-2 as an oxidant to the NH3 nitridation reaction system. Investigation of the relationship between O-2 concentration and the crystal phase of the resulting samples revealed that Nb2O5 nitridation produces crystalline NbON (Nb5+) within a specific O-2 concentration range. The O-2 concentration required to synthesize NbON increases from 0.05% to over 0.40% upon increasing the nitridation temperature from 600 degrees C to 750 degrees C. The photoelectrochemical properties of NbON films were also investigated. Under simulated sunlight illumination, a NbON photoanode produced a photocurrent of 0.6 mA cm(-2) at 1.2 V vs. RHE in H2O2 aqueous solution.