Gas cleaning at high temperatures, as it is required in new combined gas and steam turbine processes on the basis of coal still is one of the major open tasks. Here, the flue gas has to be cleaned at the operating temperature and pressure. Conventional separation techniques (granular bed filters) are still not able to achieve the limiting values of particle size and mass concentration for a secure operation of the turbines. Therefore, new separation techniques have to be consulted. The fact that at these temperatures the particles are highly charged due to thermionisation may enable the realization of a separation technique based on electrostatic precipitation at high temperatures. Therefore, the size dependent grade efficiencies of fine slag particles (Al-Si) had been determined. The results have shown, that separation of particles in the micron and submicron size range is possible. Furthermore, the separation efficiency was found to be virtually independent of the particle size.