Thin films of Ag and Au were deposited on vacuum‐cleaved alkali halides in ultrahigh vacuum with and without controlled‐chlorine exposures. Chlorine exposures were made (1) prior to deposition, (2) continuously during deposition, or (3) with various delay times after the start of deposition. The film structure and orientation was studied by conventional electron diffraction and electron microscopy. Results show that chlorine induces nearly perfect epitaxy of Ag and Au on NaCl and enhances epitaxy of Ag and Au on KCl and Kbr under deposition conditions that otherwise would produce polycrystalline films. For example, epitaxy of Au on NaCl is obtained at room temperature in a partial pressure of ∼5×10−7Torr Cl2. Possible mechanisms responsible for the influence of Cl2on the orientation of Ag and Au are discussed.