The structure of Rh-Cr mixed-oxide (Rh2-yCryO3) nanoparticles dispersed on (Ga1-xZnx)(N1-xOx) is characterized by electron microscopy and X-ray spectroscopy. The Rh2-yCryO3 nanoparticle is an efficient cocatalyst for photocatalytic overall water splitting on the (Ga1-xZnx)(N1-xOx) solid solution and is loaded onto the catalyst by impregnation from an aqueous solution containing Na(3)RhCl(6)(.)2H(2)O and Cr(NO3)(3)(.)9H(2)O followed by calcination in air. Impregnation of the (Ga1-xZnx)(N1-xOx) with 1 wt % Rh and 1.5 wt % Cr followed by calcination at 623 K for 1 h provides the highest photocatalytic activity. Structural analyses reveal that the activity of this photocatalyst is strongly dependent on the generation of trivalent Rh-Cr mixed-oxide nanoparticles with optimal composition and distribution.