Hydrogenolysis of different rank coals as Taiheiyo coal (75.7C%) and Pittston coal (85.4C%) were conducted in the presence of four kinds of catalysts (CuCrO4, Fe2O3+S, MoO3-TiO2 and MoS3-Al2O3) with an addition of tetralin as vehicle. The variation in viscosity in every reaction system was traced in the initial stage of the reaction by a torque meter attached to an autoclave. As a result, in every system, the torque decreased in the initial stage of temperature rise because of reduction of a solvent viscosity. The torque subsequently increased with temperature indicating two peaks before the following gradual decrease. In Taiheiyo coal, both peaks were low, and its coal conversion was more than 85%, while in Pittston coal, both peaks were sharp and high, and both subsequent torque reduction and coal conversion were smaller than those of Taiheiyo coal. 5 refs., 10 figs., 2 tabs.