The photochemical properties of Mo(CO)6adsorbed on Si(100) were investigated using temperature programmed desorption (TPD) and Auger spectroscopy. TPD experiments indicate that Mo(CO)6physisorbs on silicon and desorbs at 210–230 K. At 150 K, KrF laser radiation (248 nm) partially decomposes the adsorbed Mo(CO)6releasing gas‐phase CO in the process and TPD experiments after irradiation show that additional CO desorbs at 335 K. However, Auger analysis indicates that one CO molecule per molybdenum atom dissociates, leaving the molybdenum overlayer heavily contaminated with carbon and oxygen. The cross section for photodecomposition was measured to be 5±3×10−17cm2. Decomposition of the excited molecule must compete strongly with energy relaxation to account for the magnitude of this cross section.