Aqueous dispersions of silica nanoparticles have been aggregated through the addition of Al-13 polycations and then submitted to osmotic compression. The structures of these dispersions have been determined through small-angle neutron scattering, before and after compression. Some dispersions consisted of mixtures of aggregated and nonaggregated particles-actually a few aggregates dispersed in a "sea" of nonaggregated particles. In these dispersions, it was found that the resistance to osmotic compression originated from the ionic repulsions of the nonaggregated particles; the compression law that related the applied osmotic pressure Pi to the silica volume fraction Phi was Pi similar to [Phi/(1 - Phi)](2). Other dispersions were fully aggregated, with all particles forming a fractal network that extended throughout the available volume. In these dispersions, it was found that the resistance to compression originated from surface-surface interparticle bonds. The application of low osmotic pressures (< 50 kPa) resulted in compression at macroscopic scales only (> 300 nm), while the structure of the network at local and mesoscopic scales was unchanged. Accordingly, few interparticle bonds were broken, and the deformation was primarily elastic. The compression law for this elastic deformation was in agreement with the predicted scaling law Pi similar to Phi(4). The application of higher osmotic pressures (> 50 kPa) resulted in compression at macroscopic and mesoscopic scales (30-300 nm), while the local structure was still retained. Accordingly, many more interparticle bonds were broken. The compression law for this plastic deformation was in agreement with a scaling prediction of Pi similar to Phi(1.7). The location of the elastic-plastic transition indicated that the strength of the interparticle bonds was on the order of 5 times the thermal energies at ambient temperature.