This paper details NaBH4 hydrolysis over Ru catalysts supported on activated carbons of different origin and morphology. The influence of two Ru precursors [RuCl3 or Ru(NO)(NO3)3] was also investigated. It was found that the H2 production rate strongly depends both on the support characteristics and the Ru precursor, the best performance being obtained using an activated carbon of vegetable origin as the support and Ru(NO)(NO3)3 as the precursor. It was concluded that the carbon type and the Ru precursor mutually affect the size of the supported Ru nanoparticles with the best combination being the one leading to Ru particles with diameters around 2.5 nm, which is regarded as optimal for NaBH4 hydrolysis.Graphical Abstract