The rank of Lower Cretaceous Baganuur coal from Mongolia ranges from lignite to subbituminous coal, indicating transition from biochemical to physico-chemical coalification stages. The changes in the chemical structure of coal were studied using solid-state C-13 CP/MAS NMR. The predominant change during the transition between the two coalification stages was the dramatic decrease in dihydric and/or methoxy phenols. Concurrently. protonated aromatic carbon positioned next to O-substituted carbon decreased drastically, while C-substituted aromatic carbon increased. These changes indicate that O-containing functional group was continuously replaced by C-substituent. In addition, the amount of carbonyl and oxygenated aliphatic carbons decreased. All these transformations were greatly intensified at the beginning of the physico-chemical stage. The results, based on the chemical structure of inertinite-rich coal, suggest that during the fusinitization path, small amounts of hydrogen and oxygen remained as H- and OH-substituents of condensed aromatic rings, while other functional groups were completely destroyed. During the coalification from lignite to subbituminous rank, oxygen is significantly changed compared with carbon and hydrogen due to the elimination of oxygen-containing functional groups especially from dihydric phenol, methoxyl, and carbonyl carbon. (C) 2010 Elsevier B.V. All rights reserved.