We present the studies on the structure and optical properties of bidisperse Stober silica nanoparticulate Langmuir films prepared at the air/water interface ill a Wilhelmy film balance and transferred onto glass slides using the Langmuir-Blodgett technique. Three different compositions (covered area ratios: 4:1; 1:1, and 1:4) of two bidisperse systems were used in the experiments. Bidisperse samples (B1 and B2) were prepared by mixing the appropriate amount of monodisperse sols of particles with 61 and 100 nm diameters (B1) and those with 37 and 100 run diameters (B2). By surface pressure-area isotherms and (transmission and scanning) electron microscopy images we provide information about the structure Of the films. Optical properties of the supported films were measured with UV-vis spectroscopy and the transmittance spectra were evaluated in terms of ail optical model which allows monotonous in-depth variation of the refractive index across the film. 1 We have found that the refractive index decreased from the substrate-layer interface toward the air-layer interface when the smaller particles were in majority, and increased otherwise. That would suggest that the smaller particles of each bidisperse system call be positioned at the air side of the film if they are in minority in the sample and they call be situated on the substrate if they are in majority. The scanning electron microscope images of bidisperse films supported the in-depth film Structure Suggested by optical studies.