Trimetallic unsupported NiMoW sulfide catalysts were prepared of by decomposition-sulfidation of bimetallic Mo-W thiosalts precursors impregnated with Ni. The catalytic activity of the catalysts was determined, both insitu and exsitu, in the dibenzothiophene (DBT) hydrodesulfuration (HDS) reaction. The sample with a relative composition R1 = Mo/[Mo - W] = 0.85 and R2 = Ni/[Mo + W] = 0.5 presented a substantial increment (k = 23 x 10(-7) mol/sg) With selectivity to hydrogenation by a factor of 1.24 over hydrodesulfuration. Scanning electron microscopy (SEM) micrographs showed changes in morphology as a function of R1 suggesting that distinct crystalline phases were formed. Also, X-ray diffraction (XRD) measurements of precursors and catalysts revealed that the bimetallic thiosalt structure presents the same planes of the rhombohedral structure as ammonium thiomolybdate (ATM) or ammonium thiotungstate (ATT) indicating that redistribution of Mo and W occurred preferably in specific orientations as the (0 0 2), (0 2 0) and (3 1 0). The ammonium thiomolybdate-thiotungstate (ATMW) bimetallic thiosalt with R1 = 0.85 showed very different structure from the original Mo or W single precursors, among all the samples. Extended Huckel calculations revealed that promotion Of (MO-W)S-2 precursors with Ni forming trimetallic NiMoWS catalysts after sulfidation, increases the availability of electrons over the Fermi level increasing the metallic character of the catalysts. (C) 2008 Elsevier B.V. All rights reserved.