A series of coals with various ranks were extracted with ionic liquid (IL) 1-butyl-3-methyl-imidazolium chloride (BMIC), which was found to be a very effective solvent for the extraction of coals in a previous study. The relationship between the extract yield of coal with BMIC and the quantities of oxygen-containing functional groups in lignites was investigated. Also the mechanism of extraction of Xianfeng lignite (XL) with BMIC was investigated by the diffuse reflectance spectra (DRIFT) analysis. It was found that the extract yields of coals with BMIC varied with the ranks of coals. With the increase of the oxygen content and the decrease of carbon content, extract yield of coal with BMIC significantly increased. Extract yield of lignites with BMIC increases almost linearly with the increase of carboxyl content. The extraction with BMIC significantly changes the types and the distribution of hydrogen bonds in XL. (C) 2014 Elsevier B.V. All rights reserved.