Micro-crystalline molybdenum sulfide (MoS2) Was prepared by mechanical milling as an unsupported model catalyst for hydrodesulfurization (HDS) of diesel fuel, was prepared by mechanical milling. The catalytic function of the micro-crystallite was analyzed on the basis of the data from XRD measurements, TEM observations, and activity measurements. During the activity measurements, desulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out at 320 degreesC in an autoclave initially charged with 5 MPa of hydrogen. The desulfurizing kinetics was evaluated for the unsupported MoS2 in comparison with an industrial catalyst, alumina-supported cobalt-molybdenum sulfide. The unsupported MoS2 was more able to enhance the desulfurizing kinetics for 4,6-DMDBT than the industrial catalyst under the equivalent weight of molybdenum, while the unsupported MoS2 was inferior to the industrial catalyst in desulfurizing dibenzothiophene (DBT). Since the major catalyzed reaction was the hydrogenative desulfurization for the unsupported MoS2, it was an appropriate catalyst for 4,6-DMDBT, which was desulfurized under steric hindrance. Catalyst fouling was analyzed on the basis of the data from the real HDS reaction with diesel fuel in a down-flow tubular reactor, at 330-360 degreesC with 1.5-3.0 h(-1) under 5 MPa of hydrogen. The deposited carbon and the nitrogen constituents fouled the unsupported MoS2 more seriously than was the case for the industrial catalyst. (C) 2004 Elsevier B.V. All rights reserved.