The comminution of molybdenum sulfide particles with a vibrating mill and with a media-agitating mill was investigated. For equivalent milling times, the media-agitating mill prepared smaller molybdenum sulfide particles than the vibrating mill. Median particle diameters were approximately 40 nm for molybdenum sulfide particles comminuted for 50 h with the media-agitating mill. The specific surface area of these particles was about 120 m(2)/g. In comparison, the surface area of the starting material was 5.8 m(2)/g. The layered structure of molybdenum sulfide was sustained after the milling, although the crystal stacking height relative to the edge surface area decreased. Catalytic activities of the raw and comminuted molybdenum sulfide were measured using the hydrogenation of 1-methylnaphthalene as a model reaction. The hydrogenation rate constants per weight of comminuted catalyst were much higher than those of the raw material and increased with the specific surface area. Conversely the rate constants per surface area were almost the same irrespective of both the comminution time and the comminution method. The increased catalytic activity is believed to be caused by a change in crystal structure and morphology.