The slow aggregation of monodisperse, polydisperse, and preaggregated silica nanoparticles was studied with an electrospray-scanning mobility particle sizer (ES-SMPS) and time-resolved synchrotron radiation-small-angle X-ray scattering (SR-SAXS). Aggregation was induced by varying the NaCl concentration to obtain a fixed gelation time of ∼40 min. The combination of these techniques provides a unique tool to monitor and resolve the aggregate development in detail. The monodisperse spherical particles were converted to dimers, trimers, and eventually larger clusters as the aggregation proceeded, while the polydisperse spherical particles formed large clusters at an early stage. The initial particle shape and polydispersity had profound effects on the morphology of the aggregates; spherical primary particles produced compact spherical clusters, whereas the preaggregated dispersions formed open, elongated aggregates. All dispersions produced gels that contained free primary particles well past the point of gelation. The stability of the aggregates and the gel morphology were interpreted by relating to the structure of porous gel layers around the particles.