A solid-solution of GaN and ZnO (Ga1-xZnx) (N1-xOx,), as a visible-light-driven photo-catalyst for overall water splitting, is successfully prepared using both ZnGa2O4 and a ZnGa2O4 ZnO mixture. The photo-catalytic activity of the catalyst is found to be dependent on both the Zn/Ga ratio of the starting material and the nitridation time, because both factors affect the crystallinity and atomic composition of the final (Ga1-xZnx) (N1-xOx) material. It is also revealed that the addition of ZnO promotes crystallization of the catalyst and controls the zinc concentration, thereby improving activity. By adjusting these two preparation parameters, it is possible to obtain a material with optimal crystallinity and atomic composition for overall water splitting under visible light ( gimel > 400 nm). (c) 2007 Elsevier B.V. All rights reserved.