Yinggemajianfeng lignite (YL) was subjected to thermal dissolution (TD) in a co-solvent of an ionic liquid (IL, 1-butyl-3-methyl-imidazolium chloride, 1-butyl-3-methylimidazolium tetrafluoroborate, or 1-ethyl-3-methylimidazolium acetate) and methanol at 70 degrees C. YL and treated YL (TYL) samples were analyzed with a scanning electron microscope (SEM), thermogravimetric analyzer, solid-state C-13 nuclear magnetic resonance, and Fourier transform infrared spectrometer. The results show that the yield of soluble portion from each TYL with a co-solvent is much higher than that with the IL alone due to the decrease in the solvent viscosity and increase in the interactions between the IL and YL. The particles in the TYL samples are smaller than those in YL based on the observation with the SEM, and the TYL samples have more mass loss than YL during pyrolysis, suggesting that the TD destroyed the texture and structure of YL to some extent. Compared to YL, the TYL samples have more aliphatic carbons. The physicochemical changes of the TYL samples indicate that the co-solvents are effective for treating coals and the TYL samples are more appropriate for liquefaction and pyrolysis.