We have found that the brown coal oxidized by nitric acid at 70 degrees C can be completely dissolved in tetrahydrofuran at room temperature. Utilizing this oxidized coal, we performed the temperature-programmed reaction under liquefaction conditions in several types of solvents under nitrogen atmosphere to evaluate the retrogressive reactions. More than one-half of the tetrahydrofuran-soluble fraction was converted into the THF-insoluble fractions at 240 degrees C. The changes in the solid caused by the retrogressive reaction were examined from the change in the TG curves of insoluble fractions in toluene or tetrahydrofuran. From this analysis, it was found that two types of retrogressive reactions occurred during the liquefaction of the oxidized brown coal : one is the cross-linking reaction of THF-soluble fraction by the decomposition of oxygen functional groups at low temperatures and the other is the structural change, such as polymerization of the tetrahydrofuran-insoluble fractions, at >350 degrees C. The retrogressive reactions at low temperatures were affected strongly by the physicochemical properties of the solvent, such as solubility and vapor pressure.