Abstract Several factors which play important roles in determining the molybdenum concentration profiles in the impregnation of Al 2O 3 spheres, viz., ratio of solution volume to weight of support, state of support (dry or prewetted) and stirring of solution, have been studied. The radial distribution of molybdenum was determined in all preparations, and the nature of its surface species revealed by optical diffuse reflectance, H2-reduction, and low temperature oxygen chemisorption. The amount of molybdenum entering the Al2O3 pores and the proportion of octahedrally coordinated molybdenum species were favored by using large solution volumes and stirred solutions. In the former case, the shape of molybdenum profiles did not vary substantially, while a much more homogeneous molybdenum profile was found in the latter case. The thiophene hydrodesulphurization activity was observed to increase in catalysts prepared by using low solution volumes, ammonia as competitor, dry support or stirred impregnation solutions. A good correlation between hydrodesulphurization activity and the extent of oxygen chemisorption was also found.