Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure. DOI: 10.1103/PhysRevLett.95.048302 PACS numbers: 82.70.Gg, 61.43.Hv, 62.20.Dc, 82.70.Dd Gels are dilute connected networks which are capable of supporting applied stresses; they are commonly used to control the rheological properties of complex materials. Such networks can be formed by the aggregation of colloidal particles, which occurs when an attractive interaction is induced between them. Network elasticity is highly sensitive to the connectivity and arrangement of particles in the constituent aggregates. Colloidal gels are out-ofequilibrium systems; as a result, these networks frequently display time-dependent properties, due to network restructuring. This kind of aging is modeled for generic nonequilibrium systems as an evolution toward lower energy states, as the system explores a complex energy landscape [1,2]. However, network elasticity is also dependent on the interactions between particles; therefore, time-dependent interactions may also lead to changes in network properties. For colloidal gels, it is generally assumed that interparticle attractions, such as those described by Derujaguin-LandauVerwey-Overbeek [3,4] or Asakura-Oosawa potentials [5], determine local bond elasticity, thus, in principle, limiting their strength [6,7]. In the absence of a steric stabilization layer, particle interactions can also arise from physical bonds, as a result of covalent bonding or polymer entanglements. Within this framework, time evolution of network properties can be linked to the interparticle potential, for example, through the transport of particles from secondary to primary minima [8]. Aging can also be a consequence of time-dependent physical bonding; for example, the sintering of aggregated polystyrene particles is thought to drive local shrinkage that deforms the network [9]. The strength of the local interparticle bonds will have a direct impact on the network elasticity; however, these critical effects have never been explored.In this Letter, we present measurements of the elasticity of colloidal silica gels. Surprisingly, we find that, upon gelation, their storage moduli G 0 increase as a power law in time, G 0 t 0:4 , independent of initial volume fraction . Moreover, the time evolution of the network persists long after gelation occurs, for the duration of the measurement. As a consequence, their elasticity maintains the same volume fraction dependence, G 0 3:6 , independent of time. We postulate that the stiffening of the network is a result of chemical reactions at the junctions be...