A high-temperature x-ray diffraction furnace for use with the General Electric Model XRD-3 x-ray diffraction unit has been developed. The furnace, which was designed and constructed by the author, has an angular range of 0-165° 20. A constant temperature or continuously increasing temperature may be programmed through the temperature range of 20°C to 1000°C. Data on the high-temperature modifications of illite obtained with the high-temperature x-ray diffraction furnace indicate a contraction of the (110) spacing at 300°C and expansions at 725°C and 880°C which were not identifiable using the quench method. The existence of interlayer water in the illite structure to temperatures over 800°C is indicated, based upon the continuous decrease of the (002) basal periodicity with increasing temperature. A skeleton illite structure or a high-temperature illite phase with a d-spacing of 9.87A survives the third endothermic reaction observed by differential thermal analysis and forms a phase which is stable at 900°C. / text