Carbon-supported transition metal sulfide catalysts, prepared by pore volume impregnation of an activated carbon support with aqueous solutions of first, second and third row transition metal salts (Group V-VIII), drying and in situ sulfidation, were tested in thiophene hydrodesulfurization (723 K, 0. 1 MPa). In the poisoning experiments a bed of phosphorus compound-containing carbon was placed upstream of the catalyst bed. Under the sulfiding and reaction conditions the phosphorus compound was volatilized and transported to the catalyst bed where it could affect the catalytic properties of the active metal sulfide phase. This poisoning effect was different for the thiophene conversion to hydrocarbons and butene hydrogenation, and depended on the transition metal sulfide. Group VIII transition metal Sulfides were rather resistant towards the poisoning by the phosphorus compound. Only the hydrodesulfurization activity of the carbon-supported nickel catalyst was promoted in the presence of phosphorus. This beneficial effect of phosphorus on nickel might atso explain the gain in activity for the commercial Al2O3-supported Ni-Mo-P catalysts.