The physical and photocatalytic properties of a novel solid solution between GaN and ZnO, (Ga1-xZnx)(Ni1-xOx), are investigated. Nitridation of a mixture of Ga2O3 and ZnO at 1123 K for 5-30 h under NH3 flow results in the formation of a (Ga1-xZnx)(N1-xOx) solid solution with x = 0.05-0.22. With increasing nitridation time, the zinc and oxygen concentrations decrease due to reduction of ZnO and volatilization of zinc, and the crystallinity and band gap energy of the product increase. The highest activity for overall water splitting is obtained for (Ga1-xZnx)(N1-xOx) with x = 0.12 after nitridation for 15 h. The crystallinity of the catalyst is also found to increase with increasing the ratio of ZnO to Ga2O3 in the starting material, resulting in an increase in activity.