Alumina, titania and zirconia supported HDS catalysts have been prepared by incipient wetness impregnation with three solutions respectively based on ammonium heptamolybdate (AHM) and cobalt nitrate, P2Mo5O236- and Co2Mo10O38H46- cobalt salts. Raman spectroscopic analysis of oxidic precursors indicate that at high Mo loadings, MoO3 and CoMoO4 are present on alumina and zirconia in the AHM based materials whereas TiO2 appears as the more dispersive support whatever the impregnating solution. The thiophene HDS performances of these catalysts have been evaluated and the results are correlated to the dispersion of the oxomolybdate species in the oxidic precursor and to the promotion of the active phase. This study shows that the Co2Mo10O38H46- cobalt salt is the most efficient precursor that leads to higher dispersion and higher promoting effect. (C) 2007 Elsevier B.V. All rights reserved.