Bulk metal phosphide catalysts with nominal composition CoxNi2P (0 <= x <= 0.34) and Co0.07MoP were investigated for the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) at 583 K and 3.0 MPa H-2. The selectivity for the direct desulfurization (DDS) of 4,6-DMDBT to the product dimethylbiphenyl (DMBP) increased to 49 and 31% over the Co0.08Ni2P and Co0.07MOP catalysts, respectively, compared with the < 3% DMBP selectivity obtained on bulk Ni2P and MoP. Analysis by X-ray photoelectron spectroscopy (XPS) showed that the addition of the Co to MoP and Ni2P resulted in a P enrichment of the catalyst surface. Among the CoxNi2P and Co0.07MOP catalysts studied, the maximum DMBP selectivity occurred for the Co0.08Ni2P catalyst; this catalyst had the highest n-propylamine uptake and lowest CO uptake and the highest P/metal atom ratio, as determined by XPS. The increased selectivity to DMBP suggests that acid sites associated with phosphorous promote skeletal isomerization of 4,6-DMDBT to yield products that readily undergo DDS on the metal sites of the CoxNi2P (0 <= x <= 0.34) and Co0.07MoP catalysts. (c) 2006 Elsevier Inc. All rights reserved.