Phosphorus-containing Mo sulfide catalysts supported on ¿-Al2O3 and activated carbon were evaluated for their thiophene HDS activities. Phosphorus was added as phosphoric acid to the carrier material prior to the molybdenum component. The thiophene HDS activity of the carbon-supported catalysts was strongly decreased by phosphorus, while alumina-supported catalysts were not poisoned by phosphorus when present at moderate contents. The structural characteristics and degree of dispersion of the sulfided carbon-supported catalysts were determined by X-ray photoelectron spectroscopy and dynamic CO chemisorption. The cause of the phosphorus poisoning could not be related to a decrease in active phase dispersion or to incomplete sulfidation of the oxidic precursor catalyst. CO chemisorption revealed that in a phosphorus-containing catalyst anion vacancies were blocked. It was suggested that phosphorus poisoning can be related to phosphine (PH3), created by reduction of phosphate, probably during the presulfiding treatment. The poisoning effect can be explained as resulting from the adsorption of phosphine on the anion vacancies. The fact that alumina-supported catalysts are not poisoned by phosphorus can be explained by the strong interaction of phosphate with the alumina support. Due to this strong interaction, phosphate will not be reduced to phosphine under the sulfiding and reaction conditions applied.