Boria-alumina mixed oxides were prepared by impregnation and used as supports for CoMo catalysts (MoO3: 13 wt%; Co: 2.3 wt%) for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). A detailed characterization of B2O3-Al2O3 carriers showed that boric acid reacts with surface At-OH groups to form a boria over-layer, stabilized via Al-O-B bridges. Using X-ray diffraction, we showed that the addition of B to the alumina carrier leads to a reduction in the formation rate of bulk alpha-CoMoO4, initially present on the surface of CoMo/Al2O3 oxide precursors prepared by co-impregnation. This can be attributed to the strong interaction between Co and the B-modified carrier, as demonstrated using X-ray photoelectron spectroscopy (XPS). Further investigations using XPS showed that, because of this seemingly stronger interaction of Co-oxide species with the B2O3(x)-Al2O3 supports, the surface concentration of Mo was also affected for both oxide and sulfide CoMo catalysts. In the meantime, the degree of reducibility of Mo (XPS fraction of Mo4+/Mo-total) did not seem to be significantly affected for boria loadings below 5 wt%. However, for higher boria loadings (7 and 10 wt%), the Mo reducibility decreases with boron content due very probably to the presence of less reducible bulk MoO3 in CoMo oxide precursors. The HDS activity of DBT and 4,6-DMDBT over CoMo/B2O3(x)-Al2O3 exhibit a maximum for B2O3 loadings around 3-5 wt%. To a first approximation, this increase in the HDS activity can be attributed to the stronger interaction of Co-species with the B-modified carrier suppressing the formation of alpha-CoMoO4, which is known to yield the formation of less active Co9S8, after sulfurization, instead of generating the more active "CoMoS" phase. For boria loadings above 3-5 wt%, the DBT and 4,6-DMDBT HDS activity of CoMo/B2O3-Al2O3 catalysts decreases due to a decrease in the dispersion and reducibility/sulfidability of Mo-supported species, as evidenced using XPS. (C) 2008 Elsevier B.V. All rights reserved.