Thermal charging of submicron and nanometer particles has been studied for model aerosols of TiO2 and SiO2 as well as Al-Si (aluminosilicate) at 1 000degreesC with a new quasi in-situ technique. The size dependence of the particle separation efficiency for electrostatic precipitation was determined. The charging state of the particles was obtained from evaluating the global Deutsch number for precipitation in an electric field applied to a laminar flow based on particle trajectory considerations. A dependence of the charging state on the particle material was found to correspond to the work functions of TiO2 and SiO2. The aluminosilicate particles were significantly less charged compared to the pure metal oxides. For all materials the precipitation was found to only weakly depend on the particle size, which is a very favorable feature for an industrial application such as process gas cleaning at high temperatures by electrostatic means. The particle charge scales to somewhat less than with the diameter squared, which would agree with the hypothesis of thermionic emission.