Hydrogen addition to the products derived from cracking kerosene over-H/ZSM-5 was observed at hydrogen pressures between 4.1-8.7 MPa and at 373-390 degrees C. At low pressures, kerosene cracking over H/ZSM-5 yielded typical cracked products : aromatics, as well as low molecular weight saturates and olefins. Endothermic reactor temperature profiles were also observed, indicative of cracking reactions. At high hydrogen partial pressures product selectivity was altered in that kerosene cracking gave high yields of low molecular weight paraffins and low yields of olefins and aromatics. Reactor temperature profiles were exothermic, indicative of hydrocracking reactions. A mechanism for acid catalyzed hydrogenation is suggested. Although hydrogenation was not observed at lower hydrogen pressures, hydrogen proved beneficial in maintaining catalyst activity at a stable level. Lost catalyst activity was restored by maintaining the catalyst under static hydrogen at 1.4 MPa and 370 degrees C for 16 h.