The conversion of two coal-derived nondistillable residua (resids) in tetralin during hydroprocessing has been examined. A novel laboratory scale batch reactor capable of operation up to 450 degrees C and 17 MPa (2500 psi) under well-defined contact times from a few seconds to 30 min or longer was used. Thermogravimetric analyses, augmented by gas chromatography and gas chromatography/mass spectrometry, were used to follow the course of the conversion. Two resids, one derived from Wyodak subbituminous coal and another from Pittsburgh bituminous coal, were found to differ in their reactivity toward conversion to soluble or lower boiling materials. In the absence of a catalyst, the insoluble resids became solubilized in tetralin to some degree. However, even at long reaction times and high temperatures there was no indication of a breakdown in molecular weight or molecular structure as shown by thermogravimetric analysis and laser desorption high resolution mass spectrometry. In the presence of a presulfided Ni/Mo on alumina catalyst there was a much higher degree of solubilization and a definite indication of molecular breakdown.