Organic solvent upgrading Indonesian lignite was performed in a 1 L autoclave under moderate temperature. The chemical structure and functional groups transformation of lignite upgraded by two organic solvents (ethanol and n-hexane) were analyzed to explore the upgrading mechanism of solvent thermal treatment by using Fourier transform infrared (FTIR) and C-13 nuclear magnetic resonance (NMR). In addition, the characteristics of pyrolysis of treated samples were investigated using thermo gravimetric (TG) to clarify the variance of pyrolysis reactivity. Results showed that the carbon content and calorific value of upgraded lignite were significantly improved, and H/C and O/C ratios of treated samples were significantly reduced with the temperature increasing. The relative percentage of carbonyl and carboxyl carbon, oxygenated aliphatic carbon and methoxyl carbon of lignite upgraded at 300 degrees C decreased by 20-30%. However, the carbon-substituted and protonated aromatic carbon at 120-135 ppm and protonated aromatic carbon at 90-120 ppm were significantly increased after lignite was upgraded by the two solvents at above 200 degrees C. These transformations indicated that oxygen-containing functional group was substituted by hydrogen or carbon-substituent as temperature increased, and were intensified at above 200 degrees C. In addition, oxygen-loss in the treated samples was attributed to the loss of carbonyl group at 175 ppm, dihydric phenol at 147 ppm, and methoxyl group at 55 ppm. The activation energy of upgraded lignite at 300 degrees C were higher than those of raw lignite and upgraded lignite at 100 and 200 degrees C, indicating the low reactivity of pyrolysis of the treated lignite with the temperature increasing. (C) 2015 Elsevier Ltd. All rights reserved.