A systematic investigation of the hydrocracking activity of Mo, CoMo, and NiMo catalysts as a function of support composition, Mo content, and Co or Ni content on supported ZrO2-SiO2 catalysts of varying composition (Zr/Zr + Si = 0.0, 0.15, 0.30, 0.45, 0.70, 1.0) carried out in a microcatalytic reactor at 400 degrees C and atmospheric pressure is presented. Varying the composition of the support, the Mo content, and the Co or Ni content was studied. Cumene cracking in the presence of hydrogen on the sulfided catalysts indicated that the supported molybdenum phase and the promoted counterparts contribute significantly to the cracking functionality of the catalysts. From the studies based on SiO2 and carbon supported catalysts it appears that it is unlikely that the observed high activities of supported MoS2 and its promoted analogues are due to increased activity as a result of reduction in coke formation on the support surface. The creation of sulfhydryl groups and their role in generating the acid function are discussed.