The aggregation and syneresis of silica particles during hydrogel polymerisation has been observed in situ for the first time with near-Angstrom resolution using a new approach based on the combined fluorescence anisotropy decay of solvated and bound dye molecules. Primary particles of mean hydrodynamic diameter similar to 1.5 nm are found to be present within 20 min of mixing sodium silicate solution and sulphuric acid. Clustering then occurs during siloxane polymerisation to produce after similar to 30 h secondary particles with a mean diameter up to similar to 4.5 nm at a growth rate which depends on silicate concentration and time to microgelation, t(g). Subsequent condensation to similar to 4 nm diameter occurs within 1 week as particle syneresis dominates. The effects on particle growth of adding D2O and inorganic salts are demonstrated.