The coloading effect of H2 and O2 evolution cocatalysts on the overall water splitting reaction was investigated using a solid solution of GaN and ZnO (hereafter termed GaN:ZnO) as a photocatalyst. GaN:ZnO was modified with nanoparticulate Mn3O4, RuO2, and IrO2 as O2 evolution cocatalysts and with core/shell-type Rh/Cr2O3 composites as H2 evolution cocatalysts. The photocatalytic activity of the coloaded samples for overall water splitting was higher than that of the samples modified with either of the O2 or H2 evolution cocatalysts alone. The activity enhancement induced by coloading was comparable for the three O2 evolution cocatalysts investigated at the optimized loading amounts. Loading of a more efficient Rh/Cr2O3 cocatalyst prepared by adsorption of Rh nanoparticles further improved the photocatalytic activity. It was concluded that a simultaneous improvement in both oxidation and reduction reactions was effective at enhancing the photocatalytic activity of GaN:ZnO, whereas the reduction reactions limited the overall reaction rate of the coloaded system more significantly.