Some low-rank coals, i.e., Wyodak coal (C%, 75.0%), acid-treated in aqueous methoxyethoxy acetic acid (MEAA) and acetic acid (AA), and extracted in polar N-methyl-2-pyrrolidinone (NMP), showed a considerable increase in thermal extraction yield at 360 degreesC, as the acid concentration increased from 0.01 to 0.1 M. No significant changes were seen with a further increase in acid concentration to 1.0 M. A corresponding decrease occurred in the intensity of FT-IR spectral bands near 1555 and 1400 cm(-1), assigned to metal carboxylate groups, as acid concentrations increased from 0.01 to 0.1 M, while bands assigned to carboxyl groups at about 1720 cm(-1) increased over the same range. Furthermore, most Mg2+ and Ca2+ ions could be removed from the coals with acids between 0.01 and 0.1 M. Thermogravimetric analyses showed that the acidtreated coal yielded a weight loss similar to that of raw coal. Thermal decomposition of acidtreated coals cannot play a significant role in the increase in extraction yield that is obtained with polar solvent. A mechanism is proposed for the processes involved in the acid treatment and thermal extraction of some low-rank coals: cation-bridging cross-links existing among metal carboxylate groups in the low-rank raw coals are released upon removal of Mg2+ and Ca2+ ions by acid treatment. The resulting carboxyl groups then form new hydrogen bonds among themselves, which can become released when polar NMP solvent is introduced. Thus, both the disruption of cation-bridging cross-links by acid treatment and the release of the hydrogen bonds by NMP are involved in the enhancement of extraction yields upon acid treatment of some low-rank coals.