Mixed Ga–Zn oxynitrides were synthesized using coprecipitation, wet-precipitation, and sol-id-solution methods. The oxynitrides were used as supports for Rh nanoparticle catalysts in photo-catalytic water splitting, CO oxidation, and H2 oxidation. Mixed Ga–Zn oxynitrides produced by wet precipitation and nitridation had good visible-light-absorption properties and high surface areas, so they were used to support uniformly sized poly(vinylpyrrolidone)-stabilized Rh nanoparticles. The nanoparticle size range was 2–9 nm. These catalysts had negligible activity in photocatalytic H2 production by water splitting with methanol as a sacrificial agent. Other mixed Ga–Zn oxynitrides were also inactive. A reference sample provided by Domen also showed very low activity. The in-fluence of particle size on Rh-catalyzed oxidation of CO and H2 was investigated. For CO oxidation, the activities of small particles were higher for particles with higher Rh oxidation degrees. The op-posite holds for H2 oxidation. Mixed Ga-Zn oxynitrides were synthesized using coprecipitation, wet-precipitation, and sol-id-solution methods. The oxynitrides were used as supports for photo-catalytic water splitting, CO oxidation, and H2 oxidation. oxynitrides produced by wet precipitation and nitridation had good visible-light-absorbing properties and high surface areas, so they were used to support uniformly sized poly (vinylpyrrolidone) -stabilized Rh nanoparticles. The nanoparticles had range from 2-9 nm. These catalysts had negligible activity in photocatalytic H2 production by water splitting with methanol as a sacrificial agent. Other mixed Ga-Zn oxynitrides were also inactive. A reference sample provided by Domen also showed very low activity. The in-fluence of particle size on Rh-catalyzed oxidation of CO and H2 was investigated. For CO oxidation, the activities of small particles were higher for particles with higher Rh oxidation degrees. The op-posite holds for H2 oxidation.