Subspecies of partially decarbonylated molybdenum hexacarbonyl supported on silica were studied by diffuse reflectance IR spectroscopy. Mo(CO)(6)/SiO2 was prepared in a fluidized bed reactor by vapor-phase adsorption of molybdenum hexacarbonyl under nitrogen flow. Decarbonylation begins when Mo(CO)(6) has adsorbed onto the silica. Dehydroxylation of the support during calcination facilitates the formation of subspecies of Mo(CO)(6). The activation energy needed for bond formation between a transition metal and silica and for decarbonylation is lower than the desorption energy of physisorbed Mo(CO)(6), and this was seen in the IR spectra as a disappearance of bands due to subspecies. When the supported Mo(CO)(6) was reheated the physisorption bands were the last to disappear from the IR spectra.