Flow reactor studies of HDS of dibenzothiophene at about 300/sup 0/C and 100 atm have shown a new dependence of catalyst activity on H/sub 2/S concentration at low values. We infer that the catalyst undergoes structural changes during reaction, sometimes becoming sulfur deficient and relatively inactive. It is evidently necessary to maintain the catalyst in a properly sulfided state during reactor startup and operation by avoiding its contacting too little H/sub 2/S. The relative activities of Ni--Mo/Al/sub 2/O/sub 3/, CO--Mo/Al/sub 2/O/sub 3/, and Ni--W/Al/sub 2/O/sub 3/ catalysts for HDN of acridine have been determined, showing that Ni--Mo/Al/sub 2/O/sub 3/ is the most active. The results are interpreted inthe context of the acridine reaction network, in which both the hydrogenation and hydrogenolysis (cracking) reactions are kinetically important.