A one-step room temperature photo-assisted synthesis has been implemented in liquid phase and under solar light for preparing highly dispersed TiO2 supported metallic Ru catalysts, with no need of final thermal treatment, external hydrogen, or chemical reductant. Whether RuCl3 chloride or Ru(acac)(3) acetylacetonate precursor salt was used, sub-nanometric metallic Ru nanoparticles were synthesized on TiO2 with a sharp size distribution, the high dispersion and the metallic nature of the nanoparticles being evidenced by transmission electron microscopy and X-ray photoelectron spectroscopy. However, the use of the chloride salt was proposed to be more suitable for preparing Ru/TiO2 catalysts, due to the lower photodeposition efficiency observed with acetylacetonate, that did not allow to synthesize Ru nanoparticles with a loading higher than 1 wt.%. Different reaction mechanisms have been proposed for explaining the behaviour of both TiO2-salt systems during the Ru nanoparticle synthesis, involving respectively, both holes and electrons charge carriers in oxidation and reduction steps with acetylacetonate, and the sole photogenerated electrons with chloride.