CoMo/Al2O3 catalysts were synthesised from the ammonium and Co salts of [CoMo6O24H6](3-) and [Co2Mo10 O38H4](6-) anions as well as Co-chelate complexes with nitrilotriacetic, ethylenediaminetetraacetic, citric and tartaric acids. The catalysts were characterised using X-ray powder diffraction, Raman spectroscopy, N-2 physisorption, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The catalysts were tested in the hydrotreating (HDT) of a mixture containing thiophene and n-hexene-1, the hydrodesulphurisation (HDS) of dibenzothiophene and 4,6-dimethyldibenzothiophene and the HDT of diesel. The type of chelating agent significantly affects the structure of the active phase and catalytic properties. The catalytic activity, selectivity and turnover frequency during the HDS and hydrogenation reactions depend on the number and location of CoMoS sites, as well as the shape of the active phase crystallites. New "structure-activity" relationships were established. These approaches yielded catalysts that demonstrated high efficiency towards the deep diesel HDT and selective HDS of a model fluid catalytic cracking gasoline. (c) 2014 Elsevier Inc. All rights reserved.