The tetrahydrofuran/carbon disulfide (THF/CS2) mixed solvent-insoluble. portion of Upper Freeport coal was subject to noncatalytic and nickel-catalyzed hydrothermal treatments under pressurized hydrogen at 300 degrees C. The petroleum ether (PE)-soluble fractions from the reaction mixtures were analyzed using gas chromatography (GC) and gas chromatography/mass spectroscopy (GC/MS). Twenty-two organic sulfur compounds (OSCs) were detected in the PE-soluble fraction from the reaction mixture of noncatalytic hydrothermal treatment, whereas only very small amounts of two OSCs were detected in the PE-soluble fraction from the reaction mixture of nickel-catalyzed hydrothermal treatment, suggesting that the thermally released OSCs from the insoluble portion were effectively removed by the nickel catalyst. The related mechanisms were discussed.