Hydrotreating nickel tungsten catalysts have been prepared by impregnation of alumina with solutions of H3PW12O40 acid and its Ni salt. It is shown that the initial heteropolyanion, its lacunar analog, nickel substituted heteropolyanion and other new formed phases are present on the surface. The thiophene conversion is related to the presence of mixed NiWS phase and to the close cooperation between WS2 and NiSx phases.