Activated-carbon (AC)-supported Ni-Mo sulfide catalysts were prepared employing ammonium thiomolybdate complexes and nickel salts as metal-sulfide precursors and a commercial AC as support. Textural and chemical characterization were carried out by XRD, TPR and BET surface-area measurements. A well-defined NiI(NH3)(6)MoS4 phase was detected in Ni-Mo/AC co-impregnated samples before pretreatments or catalytic reaction, but it disappeared or became too amorphous to be detected by XRD after prereduction or presulfiding. No relationship between the presence of the mixed phase in the precursor and the HDS activity was obvious. Although both types of pretreatments activated the samples for the hydrodesulfurization (HDS) of thiophene, presulfiding always produced higher activities than prereduction. The nickel salt used (chloride or acetate) did not affect the activity, but the type of thiomolybdate employed had a considerable influence on the HDS activity : The dimeric complex (NH4)(2)Mo2S12. 2H(2)O was a better precursor than the monomer (NH4)(2)MoS4, while the trimer (NH4)(2)Mo3S13. 1.8H(2)O was the poorer. This is probably related to the different decomposition pathways that follow the complexes, as both, the monomer and dimer, decompose to MoS2 through amorphous MoS3 while the trimer directly produces MoS2, presumably more crystalline. The monomeric thiotungstate complex (NH4)(2)WS4 produced a more active Ni-W/AC catalyst than any of the Ni-Mo/AC samples measured, although for the unpromoted samples the situation was the reverse. Comparison with the alumina-supported catalyst suggests that the thiomolybdate derived samples should be further explored for HDS.