The hydrogenation of 24 coals (LV bituminous to lignite) in the presence of Mo was studied. The hydrogenation temperatures were 300, 350 and 400°C, the hydrogen pressures 5 and 10 MPa, and the reaction time 30 min. At 300°C the Mo salts deposited on the coals do not decompose to the active MoS2 state, so the hydrogenation process is not significantly different from that in the absence of a catalyst and is influenced by the ease of diffusion, with a consequent increase in conversion with increasing macropore volume. At 350 and 400°C, MoS2 acts as a good catalyst and conversion is much greater than in the absence of a catalyst. Under these conditions, opening of pores in the coals has been detected as conversion increases; micropores develop to macropores in such a manner that the MoS2 particles can penetrate progressively into pores originally inaccessible, so an increase in the micropore volume or surface area favours conversion. This pore-opening is accompanied by an increase in cross-link density. At 5 MPa of H2, and increasing with temperature, some of the radicals formed are incorporated into the macromolecular structure by repolymerization reactions which result in blockage of the microporosity.