Platinum evaporated on a clean iridium surface at temperatures as low as 200°K produces epitaxial layers, as shown by field ion microscopy. If the iridium surface has been exposed to the atmosphere after forming, the temperature required for epitaxy rises to about 375°K (100°C). Even this temperature is well below that required for high mobility of the incoming atoms over the surface, as assumed in present theories of the formation of epitaxial layers.