The optimum conditions for a high‐temperature x‐ray diffractometer are considered. A furnace with a minimum of compromise with these optimum conditions is described in some detail. Comparisons of previously described furnaces are made. The thermal gradients in the furnace were measured by thermocouple probe at several temperatures, and a novel shielding technique (making use of the x‐ray diffraction beam) was used to establish that the thermal gradients in the furnace were indeed very low. A series of polymorphic transitions which have been studied are presented as a means of obtaining a good calibration for high‐temperature x‐ray studies.