The thiophene HDS activities of phosphorus-containing Co and Co&z,sbnd;Mo sulfide catalysts supported on ¿-Al2O3 and activated carbon were measured at atmospheric pressure. Phosphorus decreased the thiophene HDS activity of the carbon-supported catalysts considerably but did not affect the activity of the alumina-supported catalysts. From XPS it was inferred that phosphorus improves the dispersion of the cobalt phase in the oxidic precursor state of both the carbon- and alumina-supported catalysts. In the sulfided state the interaction between phosphorus and the metal sulfide phase appeared to depend on the support. In the sulfided carbon-supported catalysts, phosphorus formed a Co-phosphate phase, which is responsible for the decrease in HDS activity, while on alumina phosphorus did not influence the structure of the metal sulfide phase.