[en] Gas cooled reactors require internal thermal insulation. This is attached to the inner surface of the pressure vessels or hot ducts which are in contact with the hot gas. The main object is to protect the walls from inadmissable temperatures. Initially a metallic foil insulation and a fibre insulation have been tested. The first design of the foil insulation did not behave satisfactorily. Circumferential temperature variations of 320 K at a mean gas temperature of 660 K occured due to natural convection in gaps. Thus inadmissible thermal stress resulted in some sealing elements of the insulation. Besides the two types of insulations mentioned above ceramic materials are applied in reactor technology. For instance the hottest region below the core of the Fort St. Vrain reactor is insulated with blocks of Masrock. Furthermore rings of silica, silica nitride or kohlestein are considered for the insulation of hot gas ducts of other reactors. The advantage of this type of insulation is expected to become relevant during mounting. On this background a kohlestein-insulation has been tested in the High Pressure Wind Tunnel under air and helium conditions. The main objectives of the tests were: the efficiency of the insulation and the effect of gaps, fabrication tolerances, porosity and axial pressure gradients on the temperature distribution. These details are of importance to predict the thermal stress of the material. (Auth.)