In order to discuss the reaction process of coals treated with hydrogen, coal samples of Oyubari coal(C: 85.6%) and Taiheiy6 coal(C: 76.9%) were treated with hydrogen under mild conditions to avoid extensive degradation of the structural units. The changes in the chemical structure of benzene-solubles with changes of reaction temperature and reaction time were studied by high resolution nuclear magnetic resonance spectroscopy and elementary analysis. In addition, the chemical structure of the original coal substance was discussed on the basis of these results. As a results, it was concluded that in the initial step of hydrogen treatment up to ca.20 wt% conversion, the size of aromatic rings in the structural units of benzene-solubles became gradually larger, and the internal surface area of benzene-insoluble g also increased with progressing conversion. From these results, in this particular step it was concluded that a part of the coal substance, in which an uneven distribbtion of aromatic rings was seen in the structural units, became benzene-solubles by distillation or decomposition. In the reaction process more than 20 wt% conversion, reaction products passed through the plastic state, and the internal surface area of benzene-insolubles decreased with progressing conversion. The size of aromatic rings in the structural units of benzene-solubles was nearly constant in spite of the progress of con- version. From these results, it was concluded that there was but a limited distribution of aromatic rings in the structural units of coal substance related to this reaction step. From these results, it was demonstrated experimentally that the uneven distribution of aromatic rings in the structural units of coal was small in sub-bituminous coal (Taiheiy6 coal) and large in bituminous coal (Oyubari coal).