Hydrotreating studies with propylbenzene as a model compound on a commercial NiMo/Al2O3 catalyst at 300-360 degrees C and 6.9 MPa showed that selectivity to ring hydrogenation vs dealkylation was increased by addition of ammonia at partial pressures up to 49 kPa, although overall activity decreased. Both reactions followed first-order kinetics. Selectivity was markedly increased at the lowest temperatures but again was accompanied by the lowest overall activity. The poisoning effect of ammonia here is much greater than that previously observed an hydrodesulfurization of thiophene or hydrodeoxygenation of dibenzofuran under identical reaction conditions.