The deep hydrodesulfurization (HDS) properties of nickel phosphide (Ni 2 P) on boron-modified alumina (xBÀ Al 2 O 3) supports having different B contents have been investigated. Ni 2 P precursors were prepared on the xBÀ Al 2 O 3 supports using hypophosphite (-hypo) or phosphate (-phos) as the P source and were subsequently reduced in flowing hydrogen. The 4,6dimethyldibenzothiophene HDS activities and turnover frequencies of the Ni 2 P/xBÀ Al 2 O 3 catalysts showed a strong dependence on B loading, with a maximum observed for the hypophosphite-and phosphate-based Ni 2 P/BÀ Al 2 O 3 catalysts corresponding to 0.8 wt % and 1.2 wt % B loadings, respectively. Based on XPS and IR spectral measurements, the optimal B loadings corresponded to~20 % B 2 O 3 coverage of the γ-Al 2 O 3 support and coincided with removal of the most basic hydroxyl groups on the alumina surface. At 573 K, a Ni 2 P/0.8BÀ Al 2 O 3-hypo catalyst was 2.5 times more active than a B-free Ni 2 P/Al 2 O 3-hypo catalyst, while a Ni 2 P/1.2BÀ Al 2 O 3-phos catalyst was 8.6 times more active than a B-free Ni 2 P/Al 2 O 3-phos catalyst. Overall, the hypophosphite-based Ni 2 P/BÀ Al 2 O 3 catalysts exhibited higher HDS activities than the phosphate-based Ni 2 P/BÀ Al 2 O 3 catalysts, in part due to smaller Ni 2 P particle sizes. The optimized catalysts, Ni 2 P/0.8BÀ Al 2 O 3-hypo and Ni 2 P/1.2BÀ Al 2 O 3-phos , were more active than a sulfided NiÀ Mo/Al 2 O 3 catalyst.