The effect of boron addition was studied oil the hydrodesulfurization (HDS) of thiophene over MoS2/B/Al2O3 and Co-MoS2/B/Al2O3 (CVD-Co/MoS2/B/Al2O3), which was prepared by a CVD technique using CO(CO)(3)NO as a precursor of Co. The catalysts were characterized by means of NO adsorption and TEM. The HDS activity of MoS2/B/Al2O3 catalysts kept constant up to a boron content of about 0.6 wt% and decreased with a further increase of boron content. With CVD-Co/MoS2/B/Al2O3 catalysts, the HDS activity significantly increased as the boron content increased up to about 0.6 wt% of boron, followed by a decrease with a further increase of boron loading. Despite the activity increase, the amount of NO adsorption on MoS2/B/Al2O3 steadily decreased with increasing boron loading, suggesting that the dispersion of MoS2 particles is decreased by the addition of boron. A selective formation of the CoMoS phase on CVD-Co/MoS2/B/Al2O3 was achieved by the CVD technique. The TOF of the HDS over the CVD-Co/MoS2/B/Al2O3 catalysts, defined by the activity per Co atom forming the CoMoS phase. increased as high as 1.6 times by the addition of boron, indicating that the active phase of the catalysts shifts from less active CoMoS Type I to more active CoMoS Type II. A TEM analysis showed that the number of stacking of MoS2 slabs was only slightly increased by the addition of boron. It is concluded that the activity increase of the CVD-Co/MoS2/B/Al2O3 catalyst is caused by the formation of CoMoS Type II because of weakened interaction strength between the CoMoS phase and the Al2O3 surface by the addition of boron.