Coal structure and its evolution in catalytic hydrogenation are studied in this work. The hydrogenation experiments were performed in tubing bomb reactors, in the absence of solvent, with an initial pressure of 10 MPa of H-2, and for a reaction time of 30 min; iron sulfide was the catalyst precursor. The characterization of the solid residues was performed by different techniques, especially solvent swelling in pyridine and surface and porosity studies. The swelling values are related to the topological structure rather than to the density in noncovalent cross-linkages. The evolution of that structure is monitored by calculating the average number of carbon atoms between cross-link points, finding that at mild hydrogenation conditions (350 degrees C for low-rank coals, 400 degrees C for bituminous coals) the solid residues present a structure with longer chains of aromatic clusters than in the parent coals. This would seem to be due to the release of heteroatom-containing moieties and the prevention of cross-linking reactions by the catalyst. The textural characterization showed that there is a relationship between the increase of the skeletal density and chain length; the structure becomes more compact and ordered in the absence of low molecular weight or noncovalently bonded material.