Structural changes taking place during the hydrocracking of coal Liquefaction extracts prepared in a newing-solvent reactor (FSR) have been studied. Preparation of coal extracts in this type of reactor allows determination of hydrocracking reactivities in isolation from secondary effects associated with long product residence times during the coal dissolution step. Parallel hydrocracking experiments have been carried out with a pilot-plant (PP) extract which had a different process history. A TGA (thermogravimetric analysis)-based method for determining boiling point distributions of heavy coal-derived liquids has been described. The method has proved useful for mixtures not amenable to gas chromatography based simulated distillation. Products have also been characterized by size exclusion chromatography and UV-fluorescence spectroscopy using 1-methyl-2-pyrrolidinone as the solvent. Comparison with results obtained from these two techniques using tetrahydrofuran (THF) as solvent has indicated partial loss of sample in THF, due to poorer solubility. Compared to the pilot-plant extract, a higher proportion of the +450 degrees C bp material present in the FSR extract was found to break down in the hydrocracking stage. Parts of the +450 degrees C bp material in the extracts appear to convert to chemically more stable, large molecular-mass structures with increasing intensity and length of processing. At hydrocracking temperatures between 440 and 460 degrees C, the most significant structural changes in the FSR extracts have been observed during the first 30 min. Analogous structural differences between the PP extract and its hydrocracking products during the first 30 min were somewhat smaller. Progressively smaller changes in boiling point distributions, SEC chromatograms, and UV-F spectra have been observed between 30 and 150 min. The data do not allow distinguishing between diminishing sample reactivity and a drop in catalyst activity, but at least part of the slow-down in conversion appears due to progressive reduction in the reactivity of the extracts.