The formation and structural characteristics of Ru species applied as a cocatalyst on (Ga1-xZnx)(N1-xOx) are examined by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. RuO2 is an effective cocatalyst that enhances the activity of (Ga1-xZnx)(N1-xOx) for overall water splitting under visible-light irradiation. The highest photocatalytic activity is obtained for a sample loaded with 5.0 wt % RuO2 from an Ru-3(CO)(12) precursor followed by calcination at 623 K. Calcination is shown to cause the decomposition of initial Ru-3(CO)(12) on the (Ga1-xZnx)(N1-xOx) surface (373 K) to form Ru(IV) species (423 K). Amorphous RuO2 nanoclusters are then formed by an agglomeration of finer particles (523 K), and the nanoclusters finally crystallize (623 K) to provide the highest catalytic activity. The enhancement of catalytic activity by Ru loading from Ru-3(CO)(12) is thus shown to be dependent on the formation of crystalline RuO2 nanoparticles with optimal size and coverage.