Photoelectrochemical water splitting is a solar-to-chemical energy conversion technique. However, when n-type nonoxide semiconductors are used as photoanodes, they corrode, which is a competitive reaction with water oxidation. To evaluate these competing reactions, the optical properties (photoluminescence) of n-type GaN were examined and compared with the results of a photoelectrochemical analysis. The intermediate hole trap state associated with the yellow luminescence of GaN was found to be a candidate for the carrier transfer site for water oxidation. The hole for electrode corrosion was estimated to have a different pathway from this trap site and to be associated with the hole located at the valence band edge.