Domestic anthracite and imported bituminous coals were treated according to the molten caustic leaching procedures and changes in their organic structures were investigated. The investigation was conducted through ultimate analysis, functional group analysis using IR/NMR, and vitrinite reflectance measurements and marceral analysis with microphotometer. The results of analyses showed that the H/C ratio and the highly reactive aliphatic hydrocarbon groups linked with aromatic hydrocarbons decreased and the vitrinite reflectance increased with treatment. These are changes observed when carbonization occurs. However, the results of the observation of maceral surface showed that the surface of the organic groups was severely damaged and the anisotropy, which should increase with carbonization, decreased. The decrease in the combustion temperature and the increase in combustion reactivity were attributed to the physical changes on the surface of organic groups rather than to the chemical changes such as break down in the organic groups.