This work is the first part of a study devoted to the understanding and the determination of the molecular mechanisms that are at the origin of the specific properties shown by reinforced elastomers. Different model filled elastomers composed of cross-linked polyethylacrylate chains reinforced with grafted silica nanoparticles were prepared varying the reactivity of the coupling agent with the ethylacrylate monomers. They were synthetized applying and adapting the method developed by Ford et al. [11] which consists to polymerize a colloidal suspension of grafted silica particles in acrylate monomers. In this paper we will present how filled elastomers having different dispersion states can be prepared whilst keeping the same interactions between the particles and the polymer chains. The dispersion states were characterized by Small Angle Neutron Scattering. We found that there are two opposite effects which control the final dispersion state of these filled elastomers during the polymerization. The first one is a depletion mechanism favoring the formation of aggregates. The second one is a repulsive steric interaction due to the growth of polymer chains from the particle surfaces avoiding contacts between the silica inclusions. Using these results we can prepare sets of samples having the same particle/matrix interface but different dispersions states. By comparing their mechanical properties we should to able to estimate the relative weight of the dispersion state quality and the one of the particle/matrix interface on the mechanical behavior of these filled elastomers. (C) 2003 Elsevier Science Ltd. All rights reserved.