The design of aerocyclones operating at high temperatures is still a difficult problem since practical experience shows that the separation efficiency really obtained is mostly smaller than that calculated. For this reason, the pressure drop and the grade efficiency has been measured over the temperature range 293-1123 K. Both quantities show a remarkable influence on the gas temperature. A new model into which a wall friction coefficient depending on the temperature is introduced and consideration taken of the re-entrainment of already separated particles together with the boundary layer flow at the cyclone cover plate, the cyclone wall and along the outside of the vortex finder leads to measured pressure drops and grade efficiency curves for high temperatures of a high accuracy.