Alumina-supported Mo-based hydrodesulfurization catalysts (14 wt% nominal Mo loading) were modified by thioglycolic acid (TGA, TGA/Mo =1 mol ratio), as a chelating agent containing a thiol group. To that end, two series of catalyst were prepared: the first one by using ammonium heptamolybdate as a precursor and, in the second one, in situ obtained phosphomolybdates from MoO3 digestion in diluted H3PO4. Oxidic phases were characterized by N-2 physisorption, Raman and UV-vis spectroscopies, electron microscopy and XPS. As well, the corresponding sulfided phases were studied by XPS,HR-TEM-STEM techniques, and chemical composition was determined by STEM-EDX. Formation of ligand-to-metal charge transfer (LMCT) complexes between the S-bearing TGA and Mo atoms, resulted in reduced Mo5+ species, as indicated by XPS, and was correlated to electronic transitions bands in the UV-visible spectra. On the other hand, the catalysts prepared from phosphomolybdates impregnation and TGA addition resulted in two-fold amount of Mo4+ sulfided species, as indicated by XPS. TGA impregnation redissolve and disperse the MoO3 resulting in materials particles in materials with increased stacking of MoS2 slabs on which slabs length remained essentially unaltered. Highly active (as tested in liquid-phase dibenzothiophene hydrodesulfurization) Type II sites were favored mainly in sulfided TGA-modified formulations. (C) 2017 Elsevier B.V. All rights reserved.