Catalyst fouling during hydrocracking and conversions of larger molecular mass components have been investigated in terms of the structural features of a bituminous coal extract. The sample has been separated into two pairs of fractions: pentane-soluble (PS) and -insoluble (PI); toluene-soluble (TS) and -insoluble (TI). Differences between hydrocracked products and levels of carbon-deposition on a commercial presulfided NiMo/gamma -Al2O3 catalyst have been examined. Size exclusion chromatograms (SEC) showed MM-distributions of the samples decreasing in the order: TI > PI > TS > PS. This trend closely paralleled those given by TGA-derived boiling point distributions and the ordering of UV-fluorescence (UV-F) derived spectral shifts. In SEC, two columns with different operating ranges of molecular sizes were used. Results indicated that the largest molecular mass material did not pass through the column with the smaller molecular size range and was lost for analytical purposes. Within the range where probe mass spectrometry is capable of observation (up to similar to 600 u), the hydrocracked products of all the fractions studied contained similar ranges of molecular species; in contrast with data from TGA, SEC, and UV-F. The differences between hydrocracked products from different fractions were confined to masses beyond the range of detection by probe mass spectrometry. A reliable correspondence was found between catalyst fouling levels and the concentration of > 450 degreesC bp material in the feed. Our results are consistent with a model of the larger extract molecules, where large (> 300 u) polycyclic. aromatic (PCA) ring systems are embedded within a matrix held together by several different structural types of bridges. During hydrocracking, bridging structures between PCA ring systems break down although most PCA ring systems remain unaltered. It is thought that larger PCA groups liberated by the hydrocracking process are more likely to deposit on catalyst surfaces.