Aggregation behavior of silica nanoparticles in aqueous electrolyte solution has been studied by small-angle neutron scattering. It is found that there is local crowding of nanoparticles with increase in electrolyte concentration followed by the formation of permanent clusters above the critical salt concentration. The formation of these structures is explained on the basis of two minima observed in DLVO theory with secondary minimum and primary minimum resulting in local crowding and particle clustering, respectively. The role of varying nanoparticle size and counterions has also been examined, where aggregation follows similar behavior but can be tuned by varying these parameters. (C) 2012 Elsevier B. V. All rights reserved.