Zao Zhuang, Shin-Yubari and Upper Freeport bituminous coals were extracted with a carbon disulfide/N-methyl-2-pyrrolidinone (CS2/ NMP) mixed solvent at room temperature. Extracts were further fractionated with acetone to give acetone-soluble (AS) and acetone-insoluble (AI) fractions; the AI fraction was further extracted with pyridine to give pyridine-soluble (PS) and pyridine insoluble (PI) fractions. Despite the fact that they were part of the whole solute in the original extract, the PI fractions from all coals were only partially soluble in the mixed solvent. Combination of the AS and PS, which are lighter fractions than the PI, from the parent coal with the respective PI fraction greatly enhanced the solubility of PI in the mixed solvent. In addition, the solubility in mixed solvent of the PI fraction from Zao Zhuang coal was increased by addition of AS and PS fractions from Miike, Shin-Yubari, and Illinois No. 6 coals; the amount of enhanced solubility in comparison with the solubility of PI alone depended on the coal fraction used, i.e. AS and PS. In general, the enhancement caused by PS fractions was greater than that by AS fractions. The mechanism of solubility enhancement of the heavy constituent (PI fraction) by the addition of lighter constituents (AS and/or PS) is discussed, from the viewpoint of differences in the chemical structures of the constituents, i.e. AS, PS and PI fractions. (C) 2000 Elsevier Science Ltd. All rights reserved.