The crystal structure and optical properties of (Ga0.87Zn0.13)(N0.83O0.16) were investigated. This material was demonstrated to absorb visible light at wavelengths of up to ca. 500 nm, and to exhibit photoluminescence at around 650 nm (1.9 eV) even at room temperature. The Rietveld analyses of neutron powder diffraction data of (Ga0.97Zn0.13)(N0.83O0.16) (P6(3)mc) confirmed that oxygen substitutes for nitrogen in the crystal structure, and may be responsible for the desirable optical properties of (Ga0.97Zn0.13)(N0.83O0.16) as a photocatalyst for visible light-driven overall water splitting. The nuclear density mapping revealed the structure to be free of interstitial sites and large disorder. (c) 2005 Elsevier B.V. All rights reserved.