Nanocomposites have been made by mixing soft particles (polymer latex) with hard particles (silica) in aqueous dispersions and extracting water to produce a dense film. Segregation between the two kinds of particles can be controlled, and even suppressed. The elongational modulus is strongly increased by such fillers at low deformations, and remains important at large deformations, which the samples can stand without breaking. Since the silica particles are small (200 Å), we can follow their relative displacements under stretching, by Small-Angle Neutron Scattering, through analysis and simulation of the anisotropic patterns. The latter show a crossover from affine displacements to a set of shear displacements that let the particles avoid each other at large deformations. The shear could release the localized stresses (due to polymer confinement) and dissipate more energy. In this way it may contribute to the toughness of the composite against crack propagation.Introduction. -Soft polymeric materials can be reinforced by hard inclusions called "fillers" [1,2]. Common examples are rubber reinforced by carbon black particles, and "silicone" elastomers (PDMS) reinforced by silica. The properties of interest are, at small deformations, the mechanical modulus, and, at large deformations, the resistance to tear and wear. These properties are determined by the mechanical properties of each component, by the interfacial energy [3], and by geometrical factors such as the sizes, shapes and distances of fillers [4]. However, the mechanism of reinforcement is, at present, not understood. The roles proposed for fillers include i) temporary junctions of the polymer chains [5], ii) steric restriction described by modified Einstein laws [1, 2] or more elaborated "concentrated dispersion" models [6], ramified nature of aggregates of particles [3] leading to iii) connection in a "filler network" [7], but also iv) to overlap resistance of the bushy aggregates [8].Remarkable reinforcements can be achieved with fillers that are extremely fine, e.g., nanometric: on the one hand, large increases of the modulus may be achieved, and on the other hand, the composites can still take large deformations before they rupture [1][2][3][4]9]. The origin of this toughness of nanometric composites is still in question. In this letter, we present experimental evidence for relative displacements of nanometric particles in composites that undergo large deformations. These displacements may make it possible for the material to dissipate the energy that is stored at the tip of a fracture, and thereby accommodate large deformations without rupture.