Post-calcination at moderate temperature is found to be effective for improving the activity of (Ga1-xZnx)(N1-xOx) as a photocatalyst for overall water splitting under visible light. The activity of (Ga1-xZnx)(N1-xOx) with a composition of x = 0.12 increases with post-calcination temperature up to 873 K, above which the material commences to decompose oxidatively. This activity enhancement is attributed to a reduction in the density of zinc-and/or oxygen-related defects that function as recombination centers for photogenerated electrons and holes. The degree of enhancement by post-calcination decreases with increasing ZnO composition (x), reflecting a decrease in the as-prepared defect density with increasing x. The optimized photocatalyst, with a composition of x = 0.18, loaded with Rh2-yCryO3 cocatalyst (2.5 wt% Rh, 2 wt% Cr) and post-calcined at 823 K, exhibits an apparent quantum efficiency of ca. 5.9% at 420-440 nm, which is more than two times higher than that of the previously reported catalyst (ca. 2.5% at 420-440 nm). (C) 2007 Elsevier Inc. All rights reserved.