In hydrotreating of heavy oil residua vanadium is deposited on the catalyst surface, which causes a loss in catalyst activity due to active site poisoning and pore plugging. In order to assess the role of the structure in the hydrotreating activity of vanadium, a series of alumina- and silica-supported catalysts has been investigated. The vanadium was characterised by temperature-programmed reduction and temperature-programmed sulphiding. The catalytic performance of vanadium was tested in the hydrodesulphurisation of thiophene and the hydrodemetallisation of vanadyl-tetraphenylporphyrin. Alumina-and silica-supported vanadium catalysts differ in their dispersion. Vanadium on alumina has a higher dispersion than on silica. This dispersion effect is clearly noticeable in the catalytic activity. Silica-supported catalysts are much less active in the thiophene hydrodesulphurisation and have a lower hydrogenation rate in the hydrodemetallisation of vanadyltetraphenylporphyrin than their alumina-supported counterparts. The two catalyst systems also exhibit large differences in their hydrogenation-hydrogenolysis (ring cleavage) selectivity in the hydrodemetallisation reaction. For alumina-supported vanadium catalysts a higher selectivity for hydrogenation is found than for the silica-supported catalysts. This can be understood by the presence of at least two different types of active sites. Hydrogenation reactions in the hydrodemetallisation occur on structure (dispersion) sensitive sites, whereas ring cleavage can be associated with large clusters of active phase. For both catalyst systems an identical sulphiding mechanism is found to occur. At 1273 K the catalysts are completely sulphided into V2S3, while at 673 K the catalysts are only partially sulphided and are most likely present as an oxy-sulphide.