The action of iodine on the hexacarbonyls of chromium molybdenum, and tungsten has been studied. When heated in a sealed tube with iodine at 150°, hexacarbonylchromium yields a product of indefinite composition, whilst the carbonyls of molybdenum and tungsten both produce the tri-iodide of the metal. Tungsten tri-iodide has not been reported previously. These tri-iodides are reactive and form the complexes M(Diars)2I2(Diars =o-phenylenebisdimethylarsine) and Mopy3I3.