Mo(CO) 6 was deposited on alumina by gas phase adsorption in a fluidised bed reactor. Samples were heated gradually to determine the thermal behaviour of the surface species of molybdenum carbonyls. Mo(CO) 3 (ads) was identified as the most stable species, evidently appearing as two adsorption sites of different thermal stability. The IR spectrum recorded after deposition of Co 2 (CO) 8 on Mo/alumina was the same as the IR spectrum of Co 2 (CO) 8 on alumina. After decarbonylation at 200 o C, the ν(CO) bands due to the less stable of the Mo(CO) 3 (ads) species were better resolved when Mo(CO) 6 was deposited after decarbonylation of Co 2 (CO) 8 on alumina, than when it was directly deposited on alumina. Temperature-programmed reduction demonstrated interactions between alumina supported molybdenum and cobalt species. Activity of the samples was tested in hydrodesulphurisation of thiophene using a batch reactor. n-Butane was the main product, and the thiophene conversion was found to depend on the deposition order of the metals when the concentrations of the metals were similar.