A series of oxynitride phases in the Zn−Ge−O−N solid solution, with the Zn/Ge ratio varying from 1.4 to 1.9, have been prepared according to different synthetic routes in order to determine the influence of the preparation step on the photocatalytic activity for overall water splitting. Preliminary tests were done in the presence of sacrificial agents to test separately the ability of each phase for photoreduction and photo-oxidation. Compositions with the lowest Zn/Ge ratios are active for photoreduction while compositions with a high crystallization state present a higher activity for photo-oxidation of water. Overall water splitting was achieved only with phases active for photoreduction and appears to be likely linked to a necessary condition implying a low Zn/Ge content.