This paper reports a time-resolved small-angle X-ray scattering study of in situ Stober silica synthesis, The hydrolysis reaction is initiated by rapidly mixing equal amounts of alcoholic solutions of ammonia and tetraethyl orthosilicate, using a stopped-flow device coupled to a flow-through capillary cell. Measurements covered the scattering wave vector (q) range of 0.02 less than or equal to q less than or equal to 6 nm(-1) and time (t) range of 0.1 less than or equal to t less than or equal to 1000 s. The combination of high sensitivity, low background, and high dynamic range of the experimental setup permitted observation of the primary particles of nucleation. During the entire growth process, the measured scattered intensity can be adequately described by a sphere scattering function weighted by a Schultz size distribution function. At the early stages of growth, the fitted radius increased linearly with time, subsequently crossing over to a smaller exponent of between 1/3 and 1/2. The observed behavior is consistent with an aggregation process involving primary particles of a few nanometers in size.