Effects of electrolyte addition on photocatalytic activity of (Ga1-xZnx)(N1-xOx) modified with either Rh2-yCryO3 or RuO2 nanoparticles as cocatalysts for overall water splitting under visible light (lambda > 400 nm) are investigated. The cocatalyst Rh2-yCryO3 is confirmed to selectively promote the photoreduction of H+, while RuO2 functions as both H-2 evolution sites and as efficient O-2 evolution sites. The activity of Rh2-yCryO3-loaded (Ga1-xZnx)(N1-xOx) is found to be suppressed in the presence of Cl-, which undergoes oxidation by photogenerated holes in the valence band of (Ga1-xZnx)(N1-xOx). Alkaline-and alkaline earth-metal cations in the reactant solution compensate the negative effect of Cl- to a certain extent depending on the metal cation employed. Among the electrolytes examined, the addition of an appropriate amount of NaCl or A(2)SO(4)(A = Li, Na, or K) to the reactant solution without pH control is found to increase activity by up to 75% compared to the case without additives. Direct splitting of seawater to produce H-2 and O-2 is also demonstrated using Rh2-yCryO3-loaded (Ga1-xZnx)(N1-xOx) catalyst under visible light. (C) 2008 Elsevier B.V. All rights reserved.