We have developed a new method for measuringin- situ the growth of the nanometre-size silica particles which lead to the formation of sol-gel glasses. This technique is based on the decay of fluorescence polarisation anisotropy due to Brownian rotation of dye molecules bound to the particles. Results to date give near ångstrom resolution and demonstrate the feasibility of the approach both for providing industrial quality control and helping fundamental research. Our approach has several key advantages over traditional techniques for nanometre metrology, such as small angle X-ray and neutron scattering and electron microscopy. In this paper we present silica particle growth dynamics in a hydrogel as detected by two near-infrared dyes, the effect of adding D2O on the hydrodynamic radius and the effect of salt addition.