Catalysed sodium borohydride hydrolysis is a high-potential method to produce hydrogen for portable applications. Co-B catalysts are the most chosen because they are easily prepared, cheap and efficient. The addition of small amounts of Ru produces a significant enhancement in catalytic activity. In the present work a series of Co-Ru-B catalysts with variable Ru content was prepared, isolated and characterized. The comprehension of the synergistic effect was achieved trough the incorporation of the nanostructural dimension to the study of surface and bulk chemical states of the involved atoms along the series. It was found that up to 70% (of total metal) atomic content of Ru the catalysts can be considered isostructural to the single Co-B catalyst in the nanoscale. A structural transition occurs in the case of the pure Ru-B material to produce a boron deficient material with higher nanoparticle size. This structural transition together with Co segregation and Ru dispersion play a key role when explaining a [OH-] dependent effect. The inexistence of borate layers in Ru rich catalysts is suggestive in the research for non deactivating catalysts. (C) 2012 Elsevier B.V. All rights reserved.