Silica sols were investigated by Fourier transform infrared spectroscopy (FT-IR). The influences of water content, pH, and aging were studied. It was observed that the silica network gradually formed during aging and a more linear structure was present in the sol with low water content while high-water content sol possessed a more branched structure. The hydrolysis reaction was very fast in the sols with low pH compared with that in the high pH sols. Additionally, the sols with low pH had a greater concentration of silanol groups remaining after aging when compared with those with high pH. The IR spectra provide supplementary information about the sol to gel transformation and structure evolution.