A fine tuning of the Brønsted acidity of the hydrotreatment catalyst (by boron addition) leads to progressive changes of the electronic properties of the Mo and Co–Mo sulfided phases. This change markedly improved the hydrogenation activity in HDN test as well as in HDS of refractory molecules for Mo catalysts. •Support Brønsted acidity monitoring.•Control of the sulfide site electronic properties.•Fit of the hydrogenation activity. Aluminas with different boron loadings were prepared by impregnation with H3BO3 solutions and then used to prepare pure Mo and CoMo catalysts. According to infrared (IR) spectroscopy of 2,6-dimethylpyridine, the acid properties of the alumina have been finely tuned by boron addition. The effect of alumina acidity change on the properties of sulfided Mo and CoMo has been characterized using transmission electron microscope, X-ray photoelectron spectroscopy, and IR spectroscopy of CO-adsorption as well as model compound reactions as thiophene hydrodesulfurization (HDS), 4,6-dimethyldibenzothiophene (4,6-DMDBT) HDS and 2,6-dimethylaniline (DMA) hydrodenitrogenation (HDN). The acidity change of alumina has a direct influence on the electronic properties of MoS2 and CoMoS sites but not substantially modifies the morphology and dispersion of the sulfide phase. The results point out a relationship between the Brønsted acidity of the support and the electronic properties of the MoS2 and CoMoS phase. The change of the electronic properties of the active sites has a marked positive influence on the hydrogenation activity of the active phase. The performances of the Mo and CoMo catalysts in the reactions of HDS of thiophene and 4,6-DMDBT and HDN of DMA have been related to the variations of the structural and electronic properties resulted from boron addition.