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 degrees C, the nu(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.