Novel gold/poly-(1,3-divinyl-imidazolid-2-one)/silica [poly-bisvinylethyleneurea (poly-BVU)/silica)] hybrid particles have been produced by adsorption and spontaneously occurring in situ reduction of Au3+ cations on the surface of poly-BVU/silica hybrid particles. The successful functionalization of the poly-BVU/silica particles with gold nanoclusters has been evidenced by UV/vis and XPS spectroscopy as well as scanning electron microscopy. The size of the resulting gold clusters, estimated by means of the Mie-Drude theory on the full peak width at half-maximum of the surface plasmon UV/vis absorbance, correlates with the polymer content of the poly-BVU/silica hybrid particles used for the modification. Therefore, it is possible to control the size of the gold clusters simply by adjusting the monomer/silica ratio in the polymerization process, which corresponds with the polymer content of the hybrids. (C) 2003 Elsevier Inc. All rights reserved.