A study of co-precipitated aluminum oxide-aluminum phosphate (AAP) materials as supports of Ni-Mo heavy oil upgrading catalysts has been completed. Results of both short duration (8 h) and longer duration (up to 200 h) experiments at conditions relevant to the commercial H-Oil process are reported and compared with a commercial Ni-Mo/Al2O3 catalyst. The initial activity of the Ni-Mo/AAP catalysts correlates with the catalyst average pore diameter which is deter mined by the P content of the AAP support. An optimum pore diameter of about 20 nm exists for HDM whereas for HDS a pore diameter < 10 nm is desirable. After 100 h operation the HDM conversion of the best Ni-Mo/AAP catalyst was approximately 10 percentage points greater than for the commercial catalyst. The HDS and CCR conversions were comparable over the two catalysts. The difference in performance between the catalysts is attributed primarily to the smaller pore size of the Al2O3 support compared to the AAP support. The amount of coke deposited on the Ni-Mo/AAP catalyst was less than that on the commercial catalyst, presumably due to differences in pitch conversion levels.