The optical properties of siliconnanoparticles were measured in the mid-infrared region (2–20μm). The resulting spectra show effects of light scattering as well as absorption features due to excitations of Si–O and Si–H bonds. We are able to model the obtained spectra using an effective medium approach. The nanoparticles are best described using a Si–SiOx core-shell structure. We use the vibrational modes of the oxide to determine the thickness and the stoichiometry of the oxide. Using the Rayleigh scattering limit, we can describe the measured decrease in transmitted intensity. By fitting the theoretically modeled spectrum to the experimental data, we obtain the particle size and shape. Finally, we can identify the surface optical-phonon mode of SiOx, located between the transverse- and longitudinal-optical-phonon frequencies.