The effect of Rh co-catalyst nanoparticle size for photocatalytic water splitting using graphitic carbon nitride (g-C3N4) as light absorber was investigated. Rh nanoparticles with sizes in the 4-9 nm range were synthesized and deposited on g-C3N4. The light-absorption properties of the g-C3N4 and the particle size of Rh supported on g-C3N4 were also not influenced by the catalyst synthesis procedures. Rh/C3N4 is active in the photocatalytic splitting of water using visible light. The activity for H-2 generation does not depend on Rh particle size. The results obtained point to two important design criteria for a successful photocatalyst: firstly, the surface of the semiconductor should support a sufficient number of Rh nanoparticles to remove the photogenerated electrons before their recombination with holes; secondly, the nanoparticles should be metallic in nature to catalyze the proton-electron transfer reaction to generate adsorbed H atoms. Surface oxidation of the Rh nanoparticles substantially lowers their photocatalytic activity. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.