Hybrid nanoparticles involving 10-nm silver nanoparticles (AgNPs) nucleated on unmodified rod-like cellulose nanocrystals (CNCs) were prepared by chemical reduction. H(2)O(2)used as a post-treatment induced a size-shape transition following a redox mechanism, passing from 10-nm spherical AgNPs to 300-nm triangular or prismatic NPs (AgNPrisms), where CNCs are the only stabilizers for AgNPs and AgNPrisms. We investigated the role of the H2O2/AgNP mass ratio (alpha) on AgNPs. At alpha values above 0.20, the large amount of H(2)O(2)led to extensive oxidation that produced numerous nucleation points for AgNPrisms on CNCs. On the contrary, for alpha below 0.20, primary AgNPs are only partially oxidized, releasing a reduced amount of Ag(+)ions and thus preventing the formation of AgNPrisms and reforming spherical AgNPs. While XRD and EXAFS reveal that the AgNP fcc crystal structure is unaffected by the H(2)O(2)treatment, the XANES spectra proved that the AgNP-AgNPrism transition is always associated with an increase in the metallic Ag fraction (Ag-0). In contrast, the formation of new 15-nm spherical AgNPs keeps the initial Ag-0/Ag(+)ratio unmodified. For the first time, we introduce a complete guide map for the fully-controlled preparation of aqueous dispersed AgNPs using CNC as a template.