Five different samples of Stober silica monodisperse particles show large variations in their swelling ability as well as on their chemical compositions. Nanosized particle diameters were determined under four different conditions, using suitable techniques: photon correlation spectroscopy (PCS) in water and ethanol, AFM at 25 degreesC under 55% relative humidity, high-resolution scanning electron microscopy and transmission electron microscopy, under 10(-6) mbar. The comparison of these results shows that the smaller particles are highly swollen in ethanol, to a greater extent than in water. The swelling coefficients are lower for the larger particles, with a preference for water. Evidence for changes in the chemical composition were obtained by electron energy-loss and infrared absorption spectra: the smaller particles contain detectable amounts of C-H groups, which are not detected by IR in the larger ones, and O energy-loss spectra fine structure changes continuously with particle sizes. The location of carbon constituents in the particles was determined by electron spectroscopy imaging in the transmission electron microscope (ESI-TEM): they are dispersed throughout the finer particles, but they are excluded from the core of the larger particles. The results are interpreted considering the kinetics and extent of TEOS hydrolysis dependence on base concentration and the limiting effect of ethoxy residual groups on the densification of the silica network.