Due to its outstanding mechanical properties at high temperatures and chemical stability iridium is used for demanding high temperature applications. In order to obtain materials data necessary for the design of high temperature equipment and the numerical simulation of their service performance the stress-rupture strength and creep behaviour have been investigated in a temperature range between 1650°C and 2300°C. The results of metallographic and fracture examinations (SEM) revealed that, in common with other pure metals, unalloyed iridium shows marked grain growth at high temperatures. Under these conditions, the deformation characteristics of iridium may not be entirely uniform and predictable, as will be demonstrated with examples from the creep studies. Both metallographic examination and investigations by means of scanning electron microscopy gave indications of possible causes for a significant anomaly in the creep behaviour. It is therefore advantageous for the mechanical properties if a fine-grained microstructure can be maintained even at the highest service temperatures.