Hydrogen fluoride and a mixture of hydrogen fluoride/boron trifluoride were examined as acid catalysts for solubilization of coals of different coal rank under mild reaction conditions. Hydrogen fluoride alone accelerated solubilization of the coals with toluene at 150 degrees C to some extent. Mixtures of hydrogen. fluoride and boron trifluoride in the presence of toluene at 150 degrees C for 3 h under autogenous pressure were found to solubilize all coals nearly completely, Miike bituminous coal readily produced highly soluble products in all reaction conditions, whereas the extent of solubilization of Yallourn lignite depended on reaction conditions such as temperature, stabilizer, and acidity of acid catalyst. Solubilization of Miike bituminous coal was mainly the result of depolymerization by cleavage of methylene bridges, while in Yallourn lignite, deoxygenation as well as depolymerization was required for solubilization. Most of the remaining BF3 probably formed complexes with the organic matter, but no fluoride compounds were detected in the soluble fraction with the exception of BF3. Concentration of the main ash component, Si, in the original coal greatly decreased after the reaction, but a small proportion of another major ash component, Al, remained as AlF3.