The activity of a dispersed MoS2 catalyst prepared in. situ from ammonium tetrathiomolybdate was measured in octacosane containing various amounts of Argonne Premium Illinois No. 6 coal using microautoclaves. Conversion of 1-ethylnaphthalene (1-ETN) was followed to measure the extent of hydrogenation and/or rearrangement to 2-ETN. The reactors were charged with 1200 psig of hydrogen at room temperature. After a small amount of coal was added, activity for the acid-catalyzed rearrangement was virtually extinguished. Hydrogenation activity was significantly reduced but not eliminated. The effect on hydrogenation activity was divided into two components. One component was assigned to poisoning and the other to competitive inhibition. The weight of coal required to complete the poisoning process was determined to be 6 times the weight of Mo in the catalyst. The activity that remains for hydrogenating 1-ETN after the poisoning process is complete suffered from competitive inhibition as more coal was added. The amounts of H2S or basic nitrogen compounds that might be released from the coal during liquefaction were determined to be insufficient to account for the extent of deactivation observed.