Spherical Si and SnOx nanoparticles in the size range between 3 and 30 nm have been synthesized by microwave induced decomposition of silane and gas phase condensation, respectively. They are deposited on thin metal films and investigated by electron microscopy, Auger electron, and electron energy loss spectroscopy. An analysis of the surface composition and stoichiometry reveals that the Si particles are covered with a native oxide of less than 1 nm. The energy loss spectra show features corresponding to electronic excitations in the nanoparticles due to valence band plasmons, interband transitions, and core-level ionizations. The plasmon energies are found to increase with decreasing particle diameter d as d(-1.17) for Si and d(-0.83) for SnOx. These energy shifts are related to the change of the dielectric band gap energy of the semiconductor due to quantum size effects. (c) 2006 American Vacuum Society.