Nanostructured Langmuir-Blodgett (LB) films of Stöber-silica nanoparticles have been prepared on silicon and quartz glass substrates. The deposited layers were investigated using scanning angle reflectometry and UV-Vis spectroscopy. The reflectivity and the transmittance spectra of the LB films were evaluated using a model based on thin layer optics. Film thickness and effective refractive index of the films were determined. From the refractive index values the volume fraction of the particles in the film was estimated by effective medium approach.