A spectroscopic study is reported of the adsorption and decomposition of Mo(CO)6 on a ¿-alumina surface. Initial adsorption involves both physical adsorption and dissociative chemisorption. On subsequent heating in vacuo, most physically adsorbed Mo(CO)6 is desorbed. The remainder decomposes, forming a variety of subcarbonyl species which are identified from their infrared spectra and published temperature programmed desorption data for this system. Evidence is presented for bimolecular carbonyl species in which bridging carbonyl ligands are coordinated to Lewis acid sites on the alumina support.